grub rescue on Ubuntu 18.04 with btrfs partition

any idea about what the problem might be

Not really, though I share your suspicion that grub and btrfs may not play along well.

and how to fix it?

Have you considered saving yourself this headache and moving the /boot outside of btrfs?

You could carve out a bit of space (shrink down the swap partition by 1G - swap is overrated anyway) and get yourself a new /boot partition of this extra 1G.

For disk redundancy you can use software raid1 - grub supports it quite well.

You would lose the snapshot capability for /boot, though /boot is small enough to have some other way to recover it.