So it's a bug of the "automated remediation that makes things worse" family.
This system that checks for invalid values in the cache looks like a very bad idea in the first place as in my understanding it checks things beyond "is the cache up to date?".
A short update from meta with some initial high level technical details:
https://m.facebook.com/nt/screen/?params=%7B%22note_id%22%3A...