How do I include snapshot bootloader option in boot parameters

Did you reboot? If you still see nothing after reboot, show full /boot/grub2/grub.cfg.

I did re-boot but still no sign of required entry unless I am looking in wrong place. I had expected a line in the boot menu referring to snapshots but there is no such line. Here is my grub.cfg file:-

HP-Z640-1:/boot/grub2 # cat grub.cfg
#
# DO NOT EDIT THIS FILE
#
# It is automatically generated by grub2-mkconfig using templates
# from /etc/grub.d and settings from /etc/default/grub
#

### BEGIN /etc/grub.d/00_header ###
set btrfs_relative_path="y"
export btrfs_relative_path
if [ -f ${config_directory}/grubenv ]; then
  load_env -f ${config_directory}/grubenv
elif [ -s $prefix/grubenv ]; then
  load_env
fi

if [ "${env_block}" ] ; then
  set env_block="(${root})${env_block}"
  export env_block
  load_env -f "${env_block}"
fi

if [ "${next_entry}" ] ; then
   set default="${next_entry}"
   set next_entry=
   save_env next_entry
   if [ "${env_block}" ] ; then
     save_env -f "${env_block}" next_entry
   fi
   set boot_once=true
else
   set default="${saved_entry}"
fi

if [ x"${feature_menuentry_id}" = xy ]; then
  menuentry_id_option="--id"
else
  menuentry_id_option=""
fi

export menuentry_id_option

if [ "${prev_saved_entry}" ]; then
  set saved_entry="${prev_saved_entry}"
  save_env saved_entry
  set prev_saved_entry=
  save_env prev_saved_entry
  set boot_once=true
fi

function savedefault {
  if [ -z "${boot_once}" ]; then
    saved_entry="${chosen}"
    if [ "${env_block}" ] ; then
      save_env -f "${env_block}" saved_entry
    else
      save_env saved_entry
    fi

  fi
}

function load_video {
  if [ x$feature_all_video_module = xy ]; then
    insmod all_video
  else
    insmod efi_gop
    insmod efi_uga
    insmod ieee1275_fb
    insmod vbe
    insmod vga
    insmod video_bochs
    insmod video_cirrus
  fi
}

if [ x$feature_default_font_path = xy ] ; then
   font=unicode
else
insmod part_gpt
insmod btrfs
search --no-floppy --fs-uuid --set=root 6426a5c3-e7d1-4c54-9c71-45055b9b672d
    font="/usr/share/grub2/unicode.pf2"
fi

if loadfont $font ; then
  if [ "${grub_platform}" = "efi" ]; then
    echo "Please press 't' to show the boot menu on this console"
  fi

  set gfxmode=auto
  load_video
  insmod gfxterm
fi
terminal_input console

for i in gfxterm; do
  if [ x${use_append} = xtrue ]; then
     terminal_output --append $i
  elif terminal_output $i; then
     use_append=true;
  fi
done

insmod part_gpt
insmod btrfs
search --no-floppy --fs-uuid --set=root 6426a5c3-e7d1-4c54-9c71-45055b9b672d
insmod gfxmenu
loadfont ($root)/boot/grub2/themes/openSUSE/DejaVuSans-Bold14.pf2
loadfont ($root)/boot/grub2/themes/openSUSE/DejaVuSans10.pf2
loadfont ($root)/boot/grub2/themes/openSUSE/DejaVuSans12.pf2
loadfont ($root)/boot/grub2/themes/openSUSE/ascii.pf2
insmod png
set theme=($root)/boot/grub2/themes/openSUSE/theme.txt
export theme
if [ x${boot_once} = xtrue ]; then
  set timeout=0
elif [ x$feature_timeout_style = xy ] ; then
  set timeout_style=menu
  set timeout=8
# Fallback normal timeout code in case the timeout_style feature is
# unavailable.
else
  set timeout=8
fi
if [ -n "$extra_cmdline" ]; then
  menuentry "Help on bootable snapshot #$snapshot_num" {
    echo "Select the default entry of the snapshot boot menu."
    echo "Examine the snapshot, and if it's OK,"
    echo "   run 'snapper rollback' and reboot."
    echo "See 'System Rollback by Booting from Snapshots'"
    echo "   in the manual for more information."
    echo "  ** Hit Any Key to return to boot menu **  "
    read
  }
fi
### END /etc/grub.d/00_header ###

### BEGIN /etc/grub.d/00_tuned ###
set tuned_params=""
set tuned_initrd=""
### END /etc/grub.d/00_tuned ###

### BEGIN /etc/grub.d/10_linux ###
menuentry 'openSUSE Leap 15.5'  --class opensuse --class gnu-linux --class gnu --class os $menuentry_id_option 'gnulinux-simple-6426a5c3-e7d1-4c54-9c71-45055b9b672d' {
        load_video
        set gfxpayload=keep
        insmod gzio
        insmod part_gpt
        insmod btrfs
        search --no-floppy --fs-uuid --set=root 6426a5c3-e7d1-4c54-9c71-45055b9b672d
        echo    'Loading Linux 5.14.21-150500.55.12-default ...'
        linux   /boot/vmlinuz-5.14.21-150500.55.12-default root=UUID=6426a5c3-e7d1-4c54-9c71-45055b9b672d  ${extra_cmdline} preempt=full security=apparmor mitigations=auto
        echo    'Loading initial ramdisk ...'
        initrd  /boot/initrd-5.14.21-150500.55.12-default
}
submenu 'Advanced options for openSUSE Leap 15.5' --hotkey=1 $menuentry_id_option 'gnulinux-advanced-6426a5c3-e7d1-4c54-9c71-45055b9b672d' {
        menuentry 'openSUSE Leap 15.5, with Linux 5.14.21-150500.55.12-default' --hotkey=2 --class opensuse --class gnu-linux --class gnu --class os $menuentry_id_option 'gnulinux-5.14.21-150500.55.12-default-advanced-6426a5c3-e7d1-4c54-9c71-45055b9b672d' {
                load_video
                set gfxpayload=keep
                insmod gzio
                insmod part_gpt
                insmod btrfs
                search --no-floppy --fs-uuid --set=root 6426a5c3-e7d1-4c54-9c71-45055b9b672d
                echo    'Loading Linux 5.14.21-150500.55.12-default ...'
                linux   /boot/vmlinuz-5.14.21-150500.55.12-default root=UUID=6426a5c3-e7d1-4c54-9c71-45055b9b672d  ${extra_cmdline} preempt=full security=apparmor mitigations=auto
                echo    'Loading initial ramdisk ...'
                initrd  /boot/initrd-5.14.21-150500.55.12-default
        }
        menuentry 'openSUSE Leap 15.5, with Linux 5.14.21-150500.55.12-default (recovery mode)' --hotkey=3 --class opensuse --class gnu-linux --class gnu --class os $menuentry_id_option 'gnulinux-5.14.21-150500.55.12-default-recovery-6426a5c3-e7d1-4c54-9c71-45055b9b672d' {
                load_video
                set gfxpayload=keep
                insmod gzio
                insmod part_gpt
                insmod btrfs
                search --no-floppy --fs-uuid --set=root 6426a5c3-e7d1-4c54-9c71-45055b9b672d
                echo    'Loading Linux 5.14.21-150500.55.12-default ...'
                linux   /boot/vmlinuz-5.14.21-150500.55.12-default root=UUID=6426a5c3-e7d1-4c54-9c71-45055b9b672d  ${extra_cmdline} 
                echo    'Loading initial ramdisk ...'
                initrd  /boot/initrd-5.14.21-150500.55.12-default
        }
        menuentry 'openSUSE Leap 15.5, with Linux 5.14.21-150500.55.7-default'  --class opensuse --class gnu-linux --class gnu --class os $menuentry_id_option 'gnulinux-5.14.21-150500.55.7-default-advanced-6426a5c3-e7d1-4c54-9c71-45055b9b672d' {
                load_video
                set gfxpayload=keep
                insmod gzio
                insmod part_gpt
                insmod btrfs
                search --no-floppy --fs-uuid --set=root 6426a5c3-e7d1-4c54-9c71-45055b9b672d
                echo    'Loading Linux 5.14.21-150500.55.7-default ...'
                linux   /boot/vmlinuz-5.14.21-150500.55.7-default root=UUID=6426a5c3-e7d1-4c54-9c71-45055b9b672d  ${extra_cmdline} preempt=full security=apparmor mitigations=auto
                echo    'Loading initial ramdisk ...'
                initrd  /boot/initrd-5.14.21-150500.55.7-default
        }
        menuentry 'openSUSE Leap 15.5, with Linux 5.14.21-150500.55.7-default (recovery mode)' --hotkey=1 --class opensuse --class gnu-linux --class gnu --class os $menuentry_id_option 'gnulinux-5.14.21-150500.55.7-default-recovery-6426a5c3-e7d1-4c54-9c71-45055b9b672d' {
                load_video
                set gfxpayload=keep
                insmod gzio
                insmod part_gpt
                insmod btrfs
                search --no-floppy --fs-uuid --set=root 6426a5c3-e7d1-4c54-9c71-45055b9b672d
                echo    'Loading Linux 5.14.21-150500.55.7-default ...'
                linux   /boot/vmlinuz-5.14.21-150500.55.7-default root=UUID=6426a5c3-e7d1-4c54-9c71-45055b9b672d  ${extra_cmdline} 
                echo    'Loading initial ramdisk ...'
                initrd  /boot/initrd-5.14.21-150500.55.7-default
        }
}

### END /etc/grub.d/10_linux ###

### BEGIN /etc/grub.d/20_linux_xen ###
menuentry 'openSUSE Leap 15.5, with Xen hypervisor' --class opensuse --class gnu-linux --class gnu --class os --class xen $menuentry_id_option 'xen-gnulinux-simple-6426a5c3-e7d1-4c54-9c71-45055b9b672d' {
        insmod part_gpt
        insmod fat
        search --no-floppy --fs-uuid --set=root 1CE1-6E9C
        echo    'Loading Xen 4.17.1_04-150500.3.3 with Linux 5.14.21-150500.55.12-default ...'
        chainloader  /efi/opensuse/xen-4.17.1_04-150500.3.3.efi xen-4.17.1_04-150500.3.3.efi  config.1
}
submenu 'Advanced options for openSUSE Leap 15.5 (with Xen hypervisor)' $menuentry_id_option 'gnulinux-advanced-6426a5c3-e7d1-4c54-9c71-45055b9b672d' {
        submenu 'Xen hypervisor, version 4.17.1_04-150500.3.3' $menuentry_id_option 'xen-hypervisor-4.17.1_04-150500.3.3-6426a5c3-e7d1-4c54-9c71-45055b9b672d' {
                menuentry 'openSUSE Leap 15.5, with Xen 4.17.1_04-150500.3.3 and Linux 5.14.21-150500.55.12-default' --class opensuse --class gnu-linux --class gnu --class os --class xen $menuentry_id_option 'xen-gnulinux-5.14.21-150500.55.12-default-advanced-6426a5c3-e7d1-4c54-9c71-45055b9b672d' {
                        insmod part_gpt
                        insmod fat
                        search --no-floppy --fs-uuid --set=root 1CE1-6E9C
                        echo    'Loading Xen 4.17.1_04-150500.3.3 with Linux 5.14.21-150500.55.12-default ...'
                        chainloader  /efi/opensuse/xen-4.17.1_04-150500.3.3.efi xen-4.17.1_04-150500.3.3.efi  config.2
                }
                menuentry 'openSUSE Leap 15.5, with Xen 4.17.1_04-150500.3.3 and Linux 5.14.21-150500.55.12-default (recovery mode)' --class opensuse --class gnu-linux --class gnu --class os --class xen $menuentry_id_option 'xen-gnulinux-5.14.21-150500.55.12-default-recovery-6426a5c3-e7d1-4c54-9c71-45055b9b672d' {
                        insmod part_gpt
                        insmod fat
                        search --no-floppy --fs-uuid --set=root 1CE1-6E9C
                        echo    'Loading Xen 4.17.1_04-150500.3.3 with Linux 5.14.21-150500.55.12-default ...'
                        chainloader  /efi/opensuse/xen-4.17.1_04-150500.3.3.efi xen-4.17.1_04-150500.3.3.efi  failsafe.3
                }
                menuentry 'openSUSE Leap 15.5, with Xen 4.17.1_04-150500.3.3 and Linux 5.14.21-150500.55.7-default' --class opensuse --class gnu-linux --class gnu --class os --class xen $menuentry_id_option 'xen-gnulinux-5.14.21-150500.55.7-default-advanced-6426a5c3-e7d1-4c54-9c71-45055b9b672d' {
                        insmod part_gpt
                        insmod fat
                        search --no-floppy --fs-uuid --set=root 1CE1-6E9C
                        echo    'Loading Xen 4.17.1_04-150500.3.3 with Linux 5.14.21-150500.55.7-default ...'
                        chainloader  /efi/opensuse/xen-4.17.1_04-150500.3.3.efi xen-4.17.1_04-150500.3.3.efi  config.4
                }
                menuentry 'openSUSE Leap 15.5, with Xen 4.17.1_04-150500.3.3 and Linux 5.14.21-150500.55.7-default (recovery mode)' --class opensuse --class gnu-linux --class gnu --class os --class xen $menuentry_id_option 'xen-gnulinux-5.14.21-150500.55.7-default-recovery-6426a5c3-e7d1-4c54-9c71-45055b9b672d' {
                        insmod part_gpt
                        insmod fat
                        search --no-floppy --fs-uuid --set=root 1CE1-6E9C
                        echo    'Loading Xen 4.17.1_04-150500.3.3 with Linux 5.14.21-150500.55.7-default ...'
                        chainloader  /efi/opensuse/xen-4.17.1_04-150500.3.3.efi xen-4.17.1_04-150500.3.3.efi  failsafe.5
                }
        }
}

### END /etc/grub.d/20_linux_xen ###

### BEGIN /etc/grub.d/20_memtest86+ ###
### END /etc/grub.d/20_memtest86+ ###

### BEGIN /etc/grub.d/30_os-prober ###
menuentry 'Windows Boot Manager (on /dev/sda1)' --class windows --class os $menuentry_id_option 'osprober-efi-226A-AA59' {
        insmod part_gpt
        insmod fat
        set root='hd0,gpt1'
        if [ x$feature_platform_search_hint = xy ]; then
          search --no-floppy --fs-uuid --set=root --hint-bios=hd0,gpt1 --hint-efi=hd0,gpt1 --hint-baremetal=ahci0,gpt1  226A-AA59
        else
          search --no-floppy --fs-uuid --set=root 226A-AA59
        fi
        chainloader /efi/Microsoft/Boot/bootmgfw.efi
}
### END /etc/grub.d/30_os-prober ###

### BEGIN /etc/grub.d/30_uefi-firmware ###
menuentry 'UEFI Firmware Settings' $menuentry_id_option 'uefi-firmware' {
        fwsetup
}
### END /etc/grub.d/30_uefi-firmware ###

### BEGIN /etc/grub.d/35_fwupd ###
### END /etc/grub.d/35_fwupd ###

### BEGIN /etc/grub.d/40_custom ###
# This file provides an easy way to add custom menu entries.  Simply type the
# menu entries you want to add after this comment.  Be careful not to change
# the 'exec tail' line above.
### END /etc/grub.d/40_custom ###

### BEGIN /etc/grub.d/41_custom ###
if [ -f  ${config_directory}/custom.cfg ]; then
  source ${config_directory}/custom.cfg
elif [ -z "${config_directory}" -a -f  $prefix/custom.cfg ]; then
  source $prefix/custom.cfg
fi
### END /etc/grub.d/41_custom ###

### BEGIN /etc/grub.d/80_suse_btrfs_snapshot ###
if [ -f "/.snapshots/grub-snapshot.cfg" ]; then
  source "/.snapshots/grub-snapshot.cfg"
fi
### END /etc/grub.d/80_suse_btrfs_snapshot ###

### BEGIN /etc/grub.d/90_persistent ###
### END /etc/grub.d/90_persistent ###

### BEGIN /etc/grub.d/95_textmode ###
if [ "${grub_platform}" = "efi" ]; then
  # On EFI systems we can only have graphics *or* serial, so allow the user
  # to switch between the two
  hiddenentry 'Text mode' --hotkey 't' {
    set textmode=true
    terminal_output console
  }
fi
### END /etc/grub.d/95_textmode ###
HP-Z640-1:/boot/grub2 # 

You need to recreate grub.cfg now, after /.snapshots has been mounted (it is mounted, is not it)?

update-bootloader --refresh

AFAIK it is mounted:-

/dev/nvme0n1p4 on /.snapshots type btrfs (rw,relatime,ssd,space_cache,subvolid=652,subvol=/@/.snapshots)

Still no sign of Bootable snapshots on my boot menu after refreshing update-bootloader.

Well, show /boot/grub2/grub.cfg after it has been regenerated. Did you check timestamp (that it was regenerated)?

The refresh command works each time. I have just re-booted again but still no Bootable snapshots on boot screen. Since you have doubts ere is a refresh I just completed:-

alastair@HP-Z640-1:~> su -
Password: 
HP-Z640-1:~ # ls -l /boot/grub2/grub.cfg
-rw------- 1 root root 12108 Aug  7 11:17 /boot/grub2/grub.cfg
HP-Z640-1:~ # update-bootloader --refresh
HP-Z640-1:~ # ls -l /boot/grub2/grub.cfg
-rw------- 1 root root 12108 Aug  7 11:20 /boot/grub2/grub.cfg
HP-Z640-1:~ # 

You can see the timestamps.

Here is the cfg:-

HP-Z640-1:~ # cat /boot/grub2/grub.cfg
#
# DO NOT EDIT THIS FILE
#
# It is automatically generated by grub2-mkconfig using templates
# from /etc/grub.d and settings from /etc/default/grub
#

### BEGIN /etc/grub.d/00_header ###
set btrfs_relative_path="y"
export btrfs_relative_path
if [ -f ${config_directory}/grubenv ]; then
  load_env -f ${config_directory}/grubenv
elif [ -s $prefix/grubenv ]; then
  load_env
fi

if [ "${env_block}" ] ; then
  set env_block="(${root})${env_block}"
  export env_block
  load_env -f "${env_block}"
fi

if [ "${next_entry}" ] ; then
   set default="${next_entry}"
   set next_entry=
   save_env next_entry
   if [ "${env_block}" ] ; then
     save_env -f "${env_block}" next_entry
   fi
   set boot_once=true
else
   set default="${saved_entry}"
fi

if [ x"${feature_menuentry_id}" = xy ]; then
  menuentry_id_option="--id"
else
  menuentry_id_option=""
fi

export menuentry_id_option

if [ "${prev_saved_entry}" ]; then
  set saved_entry="${prev_saved_entry}"
  save_env saved_entry
  set prev_saved_entry=
  save_env prev_saved_entry
  set boot_once=true
fi

function savedefault {
  if [ -z "${boot_once}" ]; then
    saved_entry="${chosen}"
    if [ "${env_block}" ] ; then
      save_env -f "${env_block}" saved_entry
    else
      save_env saved_entry
    fi

  fi
}

function load_video {
  if [ x$feature_all_video_module = xy ]; then
    insmod all_video
  else
    insmod efi_gop
    insmod efi_uga
    insmod ieee1275_fb
    insmod vbe
    insmod vga
    insmod video_bochs
    insmod video_cirrus
  fi
}

if [ x$feature_default_font_path = xy ] ; then
   font=unicode
else
insmod part_gpt
insmod btrfs
search --no-floppy --fs-uuid --set=root 6426a5c3-e7d1-4c54-9c71-45055b9b672d
    font="/usr/share/grub2/unicode.pf2"
fi

if loadfont $font ; then
  if [ "${grub_platform}" = "efi" ]; then
    echo "Please press 't' to show the boot menu on this console"
  fi

  set gfxmode=auto
  load_video
  insmod gfxterm
fi
terminal_input console

for i in gfxterm; do
  if [ x${use_append} = xtrue ]; then
     terminal_output --append $i
  elif terminal_output $i; then
     use_append=true;
  fi
done

insmod part_gpt
insmod btrfs
search --no-floppy --fs-uuid --set=root 6426a5c3-e7d1-4c54-9c71-45055b9b672d
insmod gfxmenu
loadfont ($root)/boot/grub2/themes/openSUSE/DejaVuSans-Bold14.pf2
loadfont ($root)/boot/grub2/themes/openSUSE/DejaVuSans10.pf2
loadfont ($root)/boot/grub2/themes/openSUSE/DejaVuSans12.pf2
loadfont ($root)/boot/grub2/themes/openSUSE/ascii.pf2
insmod png
set theme=($root)/boot/grub2/themes/openSUSE/theme.txt
export theme
if [ x${boot_once} = xtrue ]; then
  set timeout=0
elif [ x$feature_timeout_style = xy ] ; then
  set timeout_style=menu
  set timeout=8
# Fallback normal timeout code in case the timeout_style feature is
# unavailable.
else
  set timeout=8
fi
if [ -n "$extra_cmdline" ]; then
  menuentry "Help on bootable snapshot #$snapshot_num" {
    echo "Select the default entry of the snapshot boot menu."
    echo "Examine the snapshot, and if it's OK,"
    echo "   run 'snapper rollback' and reboot."
    echo "See 'System Rollback by Booting from Snapshots'"
    echo "   in the manual for more information."
    echo "  ** Hit Any Key to return to boot menu **  "
    read
  }
fi
### END /etc/grub.d/00_header ###

### BEGIN /etc/grub.d/00_tuned ###
set tuned_params=""
set tuned_initrd=""
### END /etc/grub.d/00_tuned ###

### BEGIN /etc/grub.d/10_linux ###
menuentry 'openSUSE Leap 15.5'  --class opensuse --class gnu-linux --class gnu --class os $menuentry_id_option 'gnulinux-simple-6426a5c3-e7d1-4c54-9c71-45055b9b672d' {
        load_video
        set gfxpayload=keep
        insmod gzio
        insmod part_gpt
        insmod btrfs
        search --no-floppy --fs-uuid --set=root 6426a5c3-e7d1-4c54-9c71-45055b9b672d
        echo    'Loading Linux 5.14.21-150500.55.12-default ...'
        linux   /boot/vmlinuz-5.14.21-150500.55.12-default root=UUID=6426a5c3-e7d1-4c54-9c71-45055b9b672d  ${extra_cmdline} preempt=full security=apparmor mitigations=auto
        echo    'Loading initial ramdisk ...'
        initrd  /boot/initrd-5.14.21-150500.55.12-default
}
submenu 'Advanced options for openSUSE Leap 15.5' --hotkey=1 $menuentry_id_option 'gnulinux-advanced-6426a5c3-e7d1-4c54-9c71-45055b9b672d' {
        menuentry 'openSUSE Leap 15.5, with Linux 5.14.21-150500.55.12-default' --hotkey=2 --class opensuse --class gnu-linux --class gnu --class os $menuentry_id_option 'gnulinux-5.14.21-150500.55.12-default-advanced-6426a5c3-e7d1-4c54-9c71-45055b9b672d' {
                load_video
                set gfxpayload=keep
                insmod gzio
                insmod part_gpt
                insmod btrfs
                search --no-floppy --fs-uuid --set=root 6426a5c3-e7d1-4c54-9c71-45055b9b672d
                echo    'Loading Linux 5.14.21-150500.55.12-default ...'
                linux   /boot/vmlinuz-5.14.21-150500.55.12-default root=UUID=6426a5c3-e7d1-4c54-9c71-45055b9b672d  ${extra_cmdline} preempt=full security=apparmor mitigations=auto
                echo    'Loading initial ramdisk ...'
                initrd  /boot/initrd-5.14.21-150500.55.12-default
        }
        menuentry 'openSUSE Leap 15.5, with Linux 5.14.21-150500.55.12-default (recovery mode)' --hotkey=3 --class opensuse --class gnu-linux --class gnu --class os $menuentry_id_option 'gnulinux-5.14.21-150500.55.12-default-recovery-6426a5c3-e7d1-4c54-9c71-45055b9b672d' {
                load_video
                set gfxpayload=keep
                insmod gzio
                insmod part_gpt
                insmod btrfs
                search --no-floppy --fs-uuid --set=root 6426a5c3-e7d1-4c54-9c71-45055b9b672d
                echo    'Loading Linux 5.14.21-150500.55.12-default ...'
                linux   /boot/vmlinuz-5.14.21-150500.55.12-default root=UUID=6426a5c3-e7d1-4c54-9c71-45055b9b672d  ${extra_cmdline} 
                echo    'Loading initial ramdisk ...'
                initrd  /boot/initrd-5.14.21-150500.55.12-default
        }
        menuentry 'openSUSE Leap 15.5, with Linux 5.14.21-150500.55.7-default'  --class opensuse --class gnu-linux --class gnu --class os $menuentry_id_option 'gnulinux-5.14.21-150500.55.7-default-advanced-6426a5c3-e7d1-4c54-9c71-45055b9b672d' {
                load_video
                set gfxpayload=keep
                insmod gzio
                insmod part_gpt
                insmod btrfs
                search --no-floppy --fs-uuid --set=root 6426a5c3-e7d1-4c54-9c71-45055b9b672d
                echo    'Loading Linux 5.14.21-150500.55.7-default ...'
                linux   /boot/vmlinuz-5.14.21-150500.55.7-default root=UUID=6426a5c3-e7d1-4c54-9c71-45055b9b672d  ${extra_cmdline} preempt=full security=apparmor mitigations=auto
                echo    'Loading initial ramdisk ...'
                initrd  /boot/initrd-5.14.21-150500.55.7-default
        }
        menuentry 'openSUSE Leap 15.5, with Linux 5.14.21-150500.55.7-default (recovery mode)' --hotkey=1 --class opensuse --class gnu-linux --class gnu --class os $menuentry_id_option 'gnulinux-5.14.21-150500.55.7-default-recovery-6426a5c3-e7d1-4c54-9c71-45055b9b672d' {
                load_video
                set gfxpayload=keep
                insmod gzio
                insmod part_gpt
                insmod btrfs
                search --no-floppy --fs-uuid --set=root 6426a5c3-e7d1-4c54-9c71-45055b9b672d
                echo    'Loading Linux 5.14.21-150500.55.7-default ...'
                linux   /boot/vmlinuz-5.14.21-150500.55.7-default root=UUID=6426a5c3-e7d1-4c54-9c71-45055b9b672d  ${extra_cmdline} 
                echo    'Loading initial ramdisk ...'
                initrd  /boot/initrd-5.14.21-150500.55.7-default
        }
}

### END /etc/grub.d/10_linux ###

### BEGIN /etc/grub.d/20_linux_xen ###
menuentry 'openSUSE Leap 15.5, with Xen hypervisor' --class opensuse --class gnu-linux --class gnu --class os --class xen $menuentry_id_option 'xen-gnulinux-simple-6426a5c3-e7d1-4c54-9c71-45055b9b672d' {
        insmod part_gpt
        insmod fat
        search --no-floppy --fs-uuid --set=root 1CE1-6E9C
        echo    'Loading Xen 4.17.1_04-150500.3.3 with Linux 5.14.21-150500.55.12-default ...'
        chainloader  /efi/opensuse/xen-4.17.1_04-150500.3.3.efi xen-4.17.1_04-150500.3.3.efi  config.1
}
submenu 'Advanced options for openSUSE Leap 15.5 (with Xen hypervisor)' $menuentry_id_option 'gnulinux-advanced-6426a5c3-e7d1-4c54-9c71-45055b9b672d' {
        submenu 'Xen hypervisor, version 4.17.1_04-150500.3.3' $menuentry_id_option 'xen-hypervisor-4.17.1_04-150500.3.3-6426a5c3-e7d1-4c54-9c71-45055b9b672d' {
                menuentry 'openSUSE Leap 15.5, with Xen 4.17.1_04-150500.3.3 and Linux 5.14.21-150500.55.12-default' --class opensuse --class gnu-linux --class gnu --class os --class xen $menuentry_id_option 'xen-gnulinux-5.14.21-150500.55.12-default-advanced-6426a5c3-e7d1-4c54-9c71-45055b9b672d' {
                        insmod part_gpt
                        insmod fat
                        search --no-floppy --fs-uuid --set=root 1CE1-6E9C
                        echo    'Loading Xen 4.17.1_04-150500.3.3 with Linux 5.14.21-150500.55.12-default ...'
                        chainloader  /efi/opensuse/xen-4.17.1_04-150500.3.3.efi xen-4.17.1_04-150500.3.3.efi  config.2
                }
                menuentry 'openSUSE Leap 15.5, with Xen 4.17.1_04-150500.3.3 and Linux 5.14.21-150500.55.12-default (recovery mode)' --class opensuse --class gnu-linux --class gnu --class os --class xen $menuentry_id_option 'xen-gnulinux-5.14.21-150500.55.12-default-recovery-6426a5c3-e7d1-4c54-9c71-45055b9b672d' {
                        insmod part_gpt
                        insmod fat
                        search --no-floppy --fs-uuid --set=root 1CE1-6E9C
                        echo    'Loading Xen 4.17.1_04-150500.3.3 with Linux 5.14.21-150500.55.12-default ...'
                        chainloader  /efi/opensuse/xen-4.17.1_04-150500.3.3.efi xen-4.17.1_04-150500.3.3.efi  failsafe.3
                }
                menuentry 'openSUSE Leap 15.5, with Xen 4.17.1_04-150500.3.3 and Linux 5.14.21-150500.55.7-default' --class opensuse --class gnu-linux --class gnu --class os --class xen $menuentry_id_option 'xen-gnulinux-5.14.21-150500.55.7-default-advanced-6426a5c3-e7d1-4c54-9c71-45055b9b672d' {
                        insmod part_gpt
                        insmod fat
                        search --no-floppy --fs-uuid --set=root 1CE1-6E9C
                        echo    'Loading Xen 4.17.1_04-150500.3.3 with Linux 5.14.21-150500.55.7-default ...'
                        chainloader  /efi/opensuse/xen-4.17.1_04-150500.3.3.efi xen-4.17.1_04-150500.3.3.efi  config.4
                }
                menuentry 'openSUSE Leap 15.5, with Xen 4.17.1_04-150500.3.3 and Linux 5.14.21-150500.55.7-default (recovery mode)' --class opensuse --class gnu-linux --class gnu --class os --class xen $menuentry_id_option 'xen-gnulinux-5.14.21-150500.55.7-default-recovery-6426a5c3-e7d1-4c54-9c71-45055b9b672d' {
                        insmod part_gpt
                        insmod fat
                        search --no-floppy --fs-uuid --set=root 1CE1-6E9C
                        echo    'Loading Xen 4.17.1_04-150500.3.3 with Linux 5.14.21-150500.55.7-default ...'
                        chainloader  /efi/opensuse/xen-4.17.1_04-150500.3.3.efi xen-4.17.1_04-150500.3.3.efi  failsafe.5
                }
        }
}

### END /etc/grub.d/20_linux_xen ###

### BEGIN /etc/grub.d/20_memtest86+ ###
### END /etc/grub.d/20_memtest86+ ###

### BEGIN /etc/grub.d/30_os-prober ###
menuentry 'Windows Boot Manager (on /dev/sdb1)' --class windows --class os $menuentry_id_option 'osprober-efi-226A-AA59' {
        insmod part_gpt
        insmod fat
        set root='hd1,gpt1'
        if [ x$feature_platform_search_hint = xy ]; then
          search --no-floppy --fs-uuid --set=root --hint-bios=hd1,gpt1 --hint-efi=hd1,gpt1 --hint-baremetal=ahci1,gpt1  226A-AA59
        else
          search --no-floppy --fs-uuid --set=root 226A-AA59
        fi
        chainloader /efi/Microsoft/Boot/bootmgfw.efi
}
### END /etc/grub.d/30_os-prober ###

### BEGIN /etc/grub.d/30_uefi-firmware ###
menuentry 'UEFI Firmware Settings' $menuentry_id_option 'uefi-firmware' {
        fwsetup
}
### END /etc/grub.d/30_uefi-firmware ###

### BEGIN /etc/grub.d/35_fwupd ###
### END /etc/grub.d/35_fwupd ###

### BEGIN /etc/grub.d/40_custom ###
# This file provides an easy way to add custom menu entries.  Simply type the
# menu entries you want to add after this comment.  Be careful not to change
# the 'exec tail' line above.
### END /etc/grub.d/40_custom ###

### BEGIN /etc/grub.d/41_custom ###
if [ -f  ${config_directory}/custom.cfg ]; then
  source ${config_directory}/custom.cfg
elif [ -z "${config_directory}" -a -f  $prefix/custom.cfg ]; then
  source $prefix/custom.cfg
fi
### END /etc/grub.d/41_custom ###

### BEGIN /etc/grub.d/80_suse_btrfs_snapshot ###
if [ -f "/.snapshots/grub-snapshot.cfg" ]; then
  source "/.snapshots/grub-snapshot.cfg"
fi
### END /etc/grub.d/80_suse_btrfs_snapshot ###

### BEGIN /etc/grub.d/90_persistent ###
### END /etc/grub.d/90_persistent ###

### BEGIN /etc/grub.d/95_textmode ###
if [ "${grub_platform}" = "efi" ]; then
  # On EFI systems we can only have graphics *or* serial, so allow the user
  # to switch between the two
  hiddenentry 'Text mode' --hotkey 't' {
    set textmode=true
    terminal_output console
  }
fi
### END /etc/grub.d/95_textmode ###

I can see the snapshot reference but it does not appear on boot screen.

Because /.snapshots is not present during boot.
What is the output of

btrfs subvolume get-default /
grep ' / ' /proc/mounts

Here are the results of the command:-

alastair@HP-Z640-1:~> sudo btrfs subvolume get-default /
[sudo] password for root: 
ID 256 gen 260203 top level 5 path @
alastair@HP-Z640-1:~> grep '/' /proc/mounts
proc /proc proc rw,nosuid,nodev,noexec,relatime 0 0
sysfs /sys sysfs rw,nosuid,nodev,noexec,relatime 0 0
devtmpfs /dev devtmpfs rw,nosuid,size=4096k,nr_inodes=1048576,mode=755,inode64 0 0
securityfs /sys/kernel/security securityfs rw,nosuid,nodev,noexec,relatime 0 0
tmpfs /dev/shm tmpfs rw,nosuid,nodev,inode64 0 0
devpts /dev/pts devpts rw,nosuid,noexec,relatime,gid=5,mode=620,ptmxmode=000 0 0
tmpfs /run tmpfs rw,nosuid,nodev,size=13169720k,nr_inodes=819200,mode=755,inode64 0 0
tmpfs /sys/fs/cgroup tmpfs ro,nosuid,nodev,noexec,size=4096k,nr_inodes=1024,mode=755,inode64 0 0
cgroup2 /sys/fs/cgroup/unified cgroup2 rw,nosuid,nodev,noexec,relatime,nsdelegate 0 0
cgroup /sys/fs/cgroup/systemd cgroup rw,nosuid,nodev,noexec,relatime,xattr,name=systemd 0 0
pstore /sys/fs/pstore pstore rw,nosuid,nodev,noexec,relatime 0 0
efivarfs /sys/firmware/efi/efivars efivarfs rw,nosuid,nodev,noexec,relatime 0 0
bpf /sys/fs/bpf bpf rw,nosuid,nodev,noexec,relatime,mode=700 0 0
cgroup /sys/fs/cgroup/perf_event cgroup rw,nosuid,nodev,noexec,relatime,perf_event 0 0
cgroup /sys/fs/cgroup/blkio cgroup rw,nosuid,nodev,noexec,relatime,blkio 0 0
cgroup /sys/fs/cgroup/cpu,cpuacct cgroup rw,nosuid,nodev,noexec,relatime,cpu,cpuacct 0 0
cgroup /sys/fs/cgroup/misc cgroup rw,nosuid,nodev,noexec,relatime,misc 0 0
cgroup /sys/fs/cgroup/memory cgroup rw,nosuid,nodev,noexec,relatime,memory 0 0
cgroup /sys/fs/cgroup/net_cls,net_prio cgroup rw,nosuid,nodev,noexec,relatime,net_cls,net_prio 0 0
cgroup /sys/fs/cgroup/hugetlb cgroup rw,nosuid,nodev,noexec,relatime,hugetlb 0 0
cgroup /sys/fs/cgroup/freezer cgroup rw,nosuid,nodev,noexec,relatime,freezer 0 0
cgroup /sys/fs/cgroup/cpuset cgroup rw,nosuid,nodev,noexec,relatime,cpuset 0 0
cgroup /sys/fs/cgroup/devices cgroup rw,nosuid,nodev,noexec,relatime,devices 0 0
cgroup /sys/fs/cgroup/pids cgroup rw,nosuid,nodev,noexec,relatime,pids 0 0
cgroup /sys/fs/cgroup/rdma cgroup rw,nosuid,nodev,noexec,relatime,rdma 0 0
/dev/nvme0n1p4 / btrfs rw,relatime,ssd,space_cache,subvolid=256,subvol=/@ 0 0
systemd-1 /proc/sys/fs/binfmt_misc autofs rw,relatime,fd=30,pgrp=1,timeout=0,minproto=5,maxproto=5,direct,pipe_ino=23764 0 0
hugetlbfs /dev/hugepages hugetlbfs rw,relatime,pagesize=2M 0 0
mqueue /dev/mqueue mqueue rw,nosuid,nodev,noexec,relatime 0 0
debugfs /sys/kernel/debug debugfs rw,nosuid,nodev,noexec,relatime 0 0
tracefs /sys/kernel/tracing tracefs rw,nosuid,nodev,noexec,relatime 0 0
fusectl /sys/fs/fuse/connections fusectl rw,nosuid,nodev,noexec,relatime 0 0
configfs /sys/kernel/config configfs rw,nosuid,nodev,noexec,relatime 0 0
/dev/nvme0n1p4 /.snapshots btrfs rw,relatime,ssd,space_cache,subvolid=652,subvol=/@/.snapshots 0 0
/dev/nvme0n1p4 /boot/grub2/i386-pc btrfs rw,relatime,ssd,space_cache,subvolid=265,subvol=/@/boot/grub2/i386-pc 0 0
/dev/nvme0n1p4 /boot/grub2/x86_64-efi btrfs rw,relatime,ssd,space_cache,subvolid=264,subvol=/@/boot/grub2/x86_64-efi 0 0
/dev/nvme0n1p4 /home btrfs rw,relatime,ssd,space_cache,subvolid=263,subvol=/@/home 0 0
/dev/nvme0n1p4 /opt btrfs rw,relatime,ssd,space_cache,subvolid=262,subvol=/@/opt 0 0
/dev/nvme0n1p4 /root btrfs rw,relatime,ssd,space_cache,subvolid=261,subvol=/@/root 0 0
/dev/nvme0n1p4 /tmp btrfs rw,relatime,ssd,space_cache,subvolid=259,subvol=/@/tmp 0 0
/dev/nvme0n1p4 /usr/local btrfs rw,relatime,ssd,space_cache,subvolid=258,subvol=/@/usr/local 0 0
/dev/nvme0n1p4 /var btrfs rw,relatime,ssd,space_cache,subvolid=257,subvol=/@/var 0 0
/dev/nvme0n1p2 /boot/efi vfat rw,relatime,fmask=0022,dmask=0022,codepage=437,iocharset=iso8859-1,shortname=mixed,utf8,errors=remount-ro 0 0
/dev/sda1 /srv xfs rw,relatime,attr2,inode64,logbufs=8,logbsize=32k,noquota 0 0
tracefs /sys/kernel/debug/tracing tracefs rw,nosuid,nodev,noexec,relatime 0 0
192.168.169.131:/Photo_database /home/alastair/NAS_photo_database nfs4 rw,relatime,vers=4.1,rsize=32768,wsize=32768,namlen=255,hard,proto=tcp,timeo=600,retrans=2,sec=sys,clientaddr=192.168.169.137,local_lock=none,addr=192.168.169.131 0 0
192.168.169.134:/multimedia/AV_multimedia /home/alastair/NFS_link_to_Mastermedia nfs4 rw,relatime,vers=4.2,rsize=1048576,wsize=1048576,namlen=255,hard,proto=tcp,timeo=600,retrans=2,sec=sys,clientaddr=192.168.169.137,local_lock=none,addr=192.168.169.134 0 0
192.168.169.131:/New_X3400_Home_Backup /home/alastair/NFS_Link_to_NAS_131 nfs4 rw,relatime,vers=4.1,rsize=32768,wsize=32768,namlen=255,hard,proto=tcp,timeo=600,retrans=2,sec=sys,clientaddr=192.168.169.137,local_lock=none,addr=192.168.169.131 0 0
binfmt_misc /proc/sys/fs/binfmt_misc binfmt_misc rw,nosuid,nodev,noexec,relatime 0 0
KeePassXC-2.7.5-x86_64.AppImage /tmp/.mount_KeePasDmSTkB fuse.KeePassXC-2.7.5-x86_64.AppImage ro,nosuid,nodev,relatime,user_id=1000,group_id=100 0 0
tmpfs /run/user/1000 tmpfs rw,nosuid,nodev,relatime,size=6584856k,nr_inodes=1646214,mode=700,uid=1000,gid=100,inode64 0 0
appimaged-655-x86_64.AppImage /tmp/.mount_appimadfrzYe fuse.appimaged-655-x86_64.AppImage ro,nosuid,nodev,relatime,user_id=1000,group_id=100 0 0
gvfsd-fuse /run/user/1000/gvfs fuse.gvfsd-fuse rw,nosuid,nodev,relatime,user_id=1000,group_id=100 0 0
portal /run/user/1000/doc fuse.portal rw,nosuid,nodev,relatime,user_id=1000,group_id=100 0 0
KeePassXC-2.7.5-x86_64.AppImage /tmp/.mount_KeePas7dFD57 fuse.KeePassXC-2.7.5-x86_64.AppImage ro,nosuid,nodev,relatime,user_id=1000,group_id=100 0 0
pcloud /tmp/.mount_pcloudbgL3uj fuse.pcloud ro,nosuid,nodev,relatime,user_id=1000,group_id=100 0 0
pCloud.fs /home/alastair/pCloudDrive fuse rw,nosuid,nodev,relatime,user_id=1000,group_id=100 0 0
KeePassXC-2.7.5-x86_64.AppImage /tmp/.mount_KeePasLZCodU fuse.KeePassXC-2.7.5-x86_64.AppImage ro,nosuid,nodev,relatime,user_id=1000,group_id=100 0 0
alastair@HP-Z640-1:~> 

That is probably the reason, but please show

btrfs inspect-internal rootid /
btrfs inspect-internal rootid /.snapshots
stat -x "%i" /.snapshots

I wonder why is it so difficult to simply copy and paste the command if you cannot even type it in as written?

alastair@HP-Z640-1:~> sudo btrfs inspect-internal rootid /
[sudo] password for root:
256
alastair@HP-Z640-1:~> sudo btrfs inspect-internal rootid /.snapshots
652
alastair@HP-Z640-1:~> stat -x “%i” /.snapshots
stat: invalid option – ‘x’
Try ‘stat --help’ for more information.
alastair@HP-Z640-1:~>

And here is my correction. I prefer to type in as I try and understand what I am typing. Tired eyes and my error. Here is what you want:-

alastair@HP-Z640-1:~> grep ' / ' /proc/mounts
/dev/nvme0n1p4 / btrfs rw,relatime,ssd,space_cache,subvolid=256,subvol=/@ 0 0
alastair@HP-Z640-1:~> 

Sorry, typo from my side, should have beem

stat -c "%i" /.snapshots

Here is revised result:-

alastair@HP-Z640-1:~> stat -c "%i" /.snapshots
256
alastair@HP-Z640-1:~> 

So far everything is as it should be. OK, last ones

btrfs inspect-internal subvolid-resolve 256 /
btrfs inspect-internal subvolid-resolve 652 /

Looks to me what I should expect:-

HP-Z640-1:~ # btrfs inspect-internal subvolid-resolve 256 /
@
HP-Z640-1:~ # btrfs inspect-internal subvolid-resolve 652 /
@/.snapshots
HP-Z640-1:~ # 

Hope it means more to you!

Yes. OK, the last

cat /boot/efi/EFI/opensuse/grub.cfg

Seems like the problem:-

HP-Z640-1:~ # cat /boot/efi/EFI/opensuse/grub.cfg
cat: /boot/efi/EFI/opensuse/grub.cfg: No such file or directory
HP-Z640-1:~ # 

Are you using EFI for boot? So far, your configuration suggests “yes”.

efibootmgr -v
ls -lR /boot/efi

I believe so but…

HP-Z640-1:~ # efibootmgr -v
BootCurrent: 000C
Timeout: 5 seconds
BootOrder: 000C,0015,000E,000F,0009,0010,000D,000A,0011,0013,0000,0001,0002,0003,0004,0005,0006,0007,0008,000B,0012
Boot0000  Startup Menu  FvVol(cdbb7b35-6833-4ed6-9ab2-57d2acddf6f0)/FvFile(9d8243e8-8381-453d-aceb-c350ee7757ca)....ISPH
Boot0001  System Information    FvVol(cdbb7b35-6833-4ed6-9ab2-57d2acddf6f0)/FvFile(9d8243e8-8381-453d-aceb-c350ee7757ca)......ISPH
Boot0002  Bios Setup    FvVol(cdbb7b35-6833-4ed6-9ab2-57d2acddf6f0)/FvFile(9d8243e8-8381-453d-aceb-c350ee7757ca)......ISPH
Boot0003  3rd Party Option ROM Management       FvVol(cdbb7b35-6833-4ed6-9ab2-57d2acddf6f0)/FvFile(9d8243e8-8381-453d-aceb-c350ee7757ca)......ISPH
Boot0004  System Diagnostics    FvVol(cdbb7b35-6833-4ed6-9ab2-57d2acddf6f0)/FvFile(9d8243e8-8381-453d-aceb-c350ee7757ca)......ISPH
Boot0005  System Diagnostics    FvVol(cdbb7b35-6833-4ed6-9ab2-57d2acddf6f0)/FvFile(9d8243e8-8381-453d-aceb-c350ee7757ca)......ISPH
Boot0006  System Diagnostics    FvVol(cdbb7b35-6833-4ed6-9ab2-57d2acddf6f0)/FvFile(9d8243e8-8381-453d-aceb-c350ee7757ca)......ISPH
Boot0007  System Diagnostics    FvVol(cdbb7b35-6833-4ed6-9ab2-57d2acddf6f0)/FvFile(9d8243e8-8381-453d-aceb-c350ee7757ca)......ISPH
Boot0008  Boot Menu     FvVol(cdbb7b35-6833-4ed6-9ab2-57d2acddf6f0)/FvFile(9d8243e8-8381-453d-aceb-c350ee7757ca)......ISPH
Boot0009  USB:          BBS(65535,,0x0)/PciRoot(0x0)/Pci(0x1d,0x0)......ISPH
Boot000A* Samsung SSD 850 PRO 256GB     BBS(HD,Harddisk1,0x400)/PciRoot(0x0)/Pci(0x11,0x4)......ISPH
Boot000B  Network Boot  FvVol(cdbb7b35-6833-4ed6-9ab2-57d2acddf6f0)/FvFile(9d8243e8-8381-453d-aceb-c350ee7757ca)......ISPH
Boot000C* opensuse      HD(2,GPT,2a324073-6a4a-479d-8b55-835a8ed62472,0x4800,0x100000)/File(\EFI\opensuse\grubx64.efi)
Boot000D* IBA GE Slot 00C8 v1550        BBS(Network,Network1,0x0)/PciRoot(0x0)/Pci(0x19,0x0)......ISPH
Boot000E  USB:          PciRoot(0x0)/Pci(0x1d,0x0)N.....YM....R,Y.....ISPH
Boot000F* hp PLDS DVDRW DU8AESH         PciRoot(0x0)/Pci(0x1f,0x2)/Sata(0,0,0)N.....YM....R,Y.....ISPH
Boot0010* hp PLDS DVDRW DU8AESH         BBS(CDROM,CDROM1,0x400)/PciRoot(0x0)/Pci(0x1f,0x2)......ISPH
Boot0011* WD1003FBYX-23 43W7629 42C0401IBM      BBS(HD,Harddisk2,0x400)/PciRoot(0x0)/Pci(0x11,0x4)......ISPH
Boot0012  HP Recovery   FvVol(cdbb7b35-6833-4ed6-9ab2-57d2acddf6f0)/FvFile(9d8243e8-8381-453d-aceb-c350ee7757ca)......ISPH
Boot0013* NVMe Slot 3   BBS(HD,Harddisk1,0x400)/PciRoot(0x0)/Pci(0x1c,0x4)/Pci(0x0,0x0)......ISPH
Boot0015* Windows Boot Manager  HD(1,GPT,9c83f1bc-1c06-4dfb-9ff4-adda277419df,0x800,0x32000)/File(\EFI\Microsoft\Boot\bootmgfw.efi)WINDOWS.........x...B.C.D.O.B.J.E.C.T.=.{.9.d.e.a.8.6.2.c.-.5.c.d.d.-.4.e.7.0.-.a.c.c.1.-.f.3.2.b.3.4.4.d.4.7.9.5.}...a....................ISPH
HP-Z640-1:~ # ls -lR /boot/efi
/boot/efi:
total 4
drwxr-xr-x 3 root root 4096 Oct  5  2021 EFI

/boot/efi/EFI:
total 4
drwxr-xr-x 2 root root 4096 Aug  7 11:20 opensuse

/boot/efi/EFI/opensuse:
total 81004
-rwxr-xr-x 1 root root      651 Aug  7 11:20 grub.xen-files
-rwxr-xr-x 1 root root   319488 Jul 23 20:07 grubx64.efi
-rwxr-xr-x 1 root root 28213088 Aug  3 18:00 initrd-5.14.21-150500.55.12-default
-rwxr-xr-x 1 root root 28206772 Aug  3 18:00 initrd-5.14.21-150500.55.7-default
-rwxr-xr-x 1 root root 11607552 Jul 31 09:58 vmlinuz-5.14.21-150500.55.12-default
-rwxr-xr-x 1 root root 11611872 Jul 11 15:23 vmlinuz-5.14.21-150500.55.7-default
-rwxr-xr-x 1 root root     1032 Aug  7 11:20 xen-4.17.1_04-150500.3.3.cfg
-rwxr-xr-x 1 root root  2975408 May 26 10:29 xen-4.17.1_04-150500.3.3.efi
HP-Z640-1:~ # 

So you do not use Secure Boot? Please show

grep -Ev '^$|^#' /etc/sysconfig/bootloader

I wonder where do you have the kernels from. That is certainly not the standard.

Hi,
All my significant installations were done when I completed major hardware changes including adding nvme instead of hard disks. The disks are still in place, usually in RAID configuration but all 4 machines run on essentially the same nvme with btrfs and a separate XFS /home directory (as I recall, will check)
The older machines are IBM servers and the BIOS could not cope with booting to nvme so I used a workaround but I am working on an HP Z640 with your help on this thread.

I did not use secure boot on Malcolm’s advice but used EFI as recommended. Here is my bootloader:

alastair@HP-Z640-1:~> su -
Password: 
HP-Z640-1:~ # grep -Ev '^$|^#' /etc/sysconfig/bootloader
LOADER_TYPE="grub2-efi"
SECURE_BOOT="no"
TRUSTED_BOOT="no"
UPDATE_NVRAM="yes"
HP-Z640-1:~ #