There will be no solution for this in SpiralLinux until Debian fixes it, but this workaround can be manually implemented on affected systems:
https://askubuntu.com/a/1032354
#Mount the other OS that is installed on Btrfs and navigate to its root
cd /mnt/exampleotherbtrfsvolume
#create symlink for /boot
ln -s @/boot boot
#create symlink for /etc
ln -s @/etc etc
#Let grub detect other OS on Btrfs
sudo update-grub2
Debian's version of os-prober does not detect other Linux installations with Btrfs that mount the
@
subvolume to/
.There will be no solution for this in SpiralLinux until Debian fixes it, but this workaround can be manually implemented on affected systems: https://askubuntu.com/a/1032354