As mentioned in comment https://pagure.io/fedora-btrfs/project/issue/7#comment-696274, btrfs /boot can't support grubenv.

The upstream idea that has the most support is for grub.cfg to point to the grubenv, rather than the core.img assuming where it is. The high level design is described in:

https://lists.gnu.org/archive/html/grub-devel/2021-05/msg00093.html

The gist is grubenv goes after core.img when on the MBR gap, or GPT BIOS Boot. Or grubenv goes on the EFI System volume as it was before https://fedoraproject.org/wiki/Changes/UnifyGrubConfig i.e. move only grubenv back to the ESP.