I don't think this is a regression, but it does seem that occasionally a btrfs filesystem gets into a state where GRUB can't read it. This may be a problem in btrfs or it may be a problem in GRUB; at the moment I can't tell. It would be good if we could debug it before you reboot, though.
Could you please provide the same grub-probe output, but this time with -vv rather than just -v?
I don't think this is a regression, but it does seem that occasionally a btrfs filesystem gets into a state where GRUB can't read it. This may be a problem in btrfs or it may be a problem in GRUB; at the moment I can't tell. It would be good if we could debug it before you reboot, though.
Could you please provide the same grub-probe output, but this time with -vv rather than just -v?