I had a similar story on my NAS that got one btrfs path corrupt. Plopped in on the btrfs IRC, one of the devs noticed the inconsistency was one bitflip away from the right value. Incredibly they were able to give me the right commands to fix it! Got to give credit where it is due, btrfs took the safe path and refused to touch the affected directory until fixed, and has enough tooling to fix this.

I won’t blame cosmic rays but more likely dying RAM. The NAS now runs ECC memory.