Some GRUBs out there (Fedora, openSUSE) have an option that makes all
paths relative to the default subvolume of the filesystem. This can be
used to include /boot in your snapshots and roll them back without
having to regenerate grub.cfg.
However, enabling that option will break grub-btrfs, because loading the
kernel from a different snapshot requires the paths to be absolute.
To make this work, GRUB has to be told explicitly to access the root
subvolume when booting to a snapshot.
