One mention in thread not showing up in Inbox, maybe

I have one case where I can't find a mention that is in a thread listed inside my Inbox.

On this server, all other mentions across channels seem to show up correctly, before and after the mention I'm looking for that is in a thread.

I have a theory that mayyyybe thread mentions aren't being listed in Inbox.

But this is my first time noticing, so I'd have to see if it ever happens again.