Tracked it down pretty far to where it's failing in XenForo (why I have no idea, it's native functionality obviously). Sent some details over to the XF team so hopefully we'll have a fix soon.
It's definitely not caching. Straight up incorrect information in DB because when a post is made, information that should be updated isn't getting updated. New threads update it, but new posts don't. Both are supposed to update it.
Happy to hear you have it figured!