Both of you are right, I guess. Thinking back, upgrading to an unstable release of GRUB is too risky.
That said, actually I use XFS for my recovery partition (a simple Slackware64-current tree rsync-ed from a remote mirror). For root and home I use BTRFS, and I prefer to avoid splitting my root, because I don't like separate /boot partition when I don't use special configurations like LVM+LUKS.
I used XFS for root just to report the bug, so until the next stable version will be released and the package will be upgraded, I think I'll switch to another FS, like ext4.
Thank you very much for your answers, both of you.
Cheers,
Luca De Pandis