Currently Leap 15.3 recognizes mint 20.3, but not Fedora 35 on booting.
Each distro is on a separate nvme device.
If I do: sudo grub2-mkconfig -o /boot/grub2/grub.cfg
then one of the messages is
Found Fedora Linux 35 (Workstation Edition) on /dev/nvme2n1p2
But in grub.cfg there is no info about Fedora and after using yast to write the boot record, there is no option for Fedora on booting.
Hi and welcome to the Forum 
Can you show the output from the following as root user, also is this system UEFI or Legacy boot?
os-prober
{if uefi boot}
efibootmgr -v
“Fedora” is probably using “btrfs”, and using a different subvolume structure than that expected by grub2 configured for “openSUSE”.
That doesn’t solve your problem, but it perhaps explains it.
os-prober:
/dev/nvme1n1p4:Linux Mint 20.3 Una (20.3):LinuxMint:linux
/dev/nvme2n1p2:Fedora Linux 35 (Workstation Edition):Fedora:linux:
btrfs:UUID=7c2e4f20-2c10-4420-b9f4-246d4e39b196:subvol=root
/dev/sdb1:MS-DOS 5.x/6.x/Win3.1:MS-DOS:chain
sudo efibootmgr -v :
[FONT=monospace]EFI variables are not supported on this system.
running as root I get the same message.
As pointed out by another reply, I believe the Fedora btrs file system is the problem.
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Run and post full protocol of execution
START=$(date +%s)
grub2-mkconfig
journalctl --since=@$START
Another possible is that they are using different booting ie one is EFI and the other is MBR in such cases they will not see one another
Note:[FONT=monospace]EFI variables are not supported on this system. is strange how old is the system If EFI is not supported then your installs must be all MBR.
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