That sounds to me like the most likely hypothesis in this thread.

I mean, the Linux manpage itself says it's stable and can be used, with the inode number, to uniquely identify a file. http://man7.org/linux/man-pages/man2/statfs.2.html

I'm not surprised, though. Fancy copy-on-write filesystems like btrfs have some other subtle gotchas. If you allocate (like, really, not as a hole) a big file on btrfs and mmap it for write, you might see a SIGBUS upon writing because btrfs needed to recompress a block and the new compressed block didn't fit where the old one did.

I've gained a new appreciation for the predictability and simplicity of ext4.