Stuck on old kernel after zypper dup

That looks to me like a grub2-bls problem, I did run into something similar, by default it did load the oldest instead of the newest kernel.

A warning here, update-bootloader does only part of what is needed to switch from grub2-bls to grub2-efi, I did try and had a boot menu with only memtest and EFI var manager. I did eventually manage this with the instructions you can find in this topic:

Kernel backups in /boot/efi/

Particularly this reddit post has concise instructions.

On a side-note, I saw in Tumbleweed - Review of the week 2026/13:

It is now clear that the previous move to grub2-bls was a mistake. We are correcting that decision by switching modern systems to systemd-boot.

So newer installs are using systemd-boot.