I have the same problem on two of my servers (both with similar
configuration, debian buster with backports), both with a btrfs raid-1
on /root.
In my case, initramfs indeed fails to mount /root, but performing a
'btrfs device scan' followed by a manual mount to /root directly on the
initramfs prompts solves the problem.
However, this problem is not systematically met. Even on the same
server, the root may be correctly mounted after a reboot.
Maybe there is a race condition, so that the
'/usr/share/initramfs-tools/scripts/local-premount/btrfs' may not be
launched at the right moment?
Hoping this information can help.
