Short update:
1) turned out to not be btrfs fault in any way or form, as we recreated
the same issue with ext4 while manually checksumming the files; so if
anything, btrfs told us we have actual issues somewhere =)
2) qemu/vm scenario is also not to be blamed, as we recreated the issue
directly on the host as well
So as far as I can see, both of the above narrows the potential culprits
to either faulty/buggy hardware/firmware somewhere - or - some subtle
lvm/md/kernel issues. Though so far pinpointing the issue is proving
rather frustrating.
Anyway, sorry for the noise.
