It's a race condition, so it had to do with two operations happening in a certain incorrect order. I bet they are able to see the exact situations where it occu

It's a race condition, so it had to do with two operations happening in a certain incorrect order. I bet they are able to see the exact situations where it occurred with pretty great certainty by reviewing the logs of the two operations that raced and see where the incorrect operation won. They even say they are invalidating all sessions or of an abundance of caution, indicative that they are pretty sure about it.