SUSE grub has patches to "mount" this subvolume at boot time and the
grub configuration file as generated by SUSE will include commands to
do it.

Thanks Andrei, I really appreciate that info as it explains why other distros cannot boot when it is a subvolume.

I looked in my TW /boot/grub2/grub.cfg but don't see references to mount the /boot/grub2/x86_64-efi subvolume.

I also grep'd in /etc/grub.d/* but still didn't find where it does that mount.

Can you give me a pointer was to where you are referring in the grub configuration?

Thanks!