O.k., to comment on that - we’re relative safe. Even if in the end it comes out that there is a bug in qemu that affects a btrfs filesystem, the Librems are by default not using btrfs.
I wish you good luck in searching for the root of the problem and just by intuition I’d point in the direction of btrfs (not saying that it is broken or bad, just a feeling and just deducted from the fact that a crashing process could compromise a whole filesystem?).