The fellowship members who are logged into the game SHOULD be listed, in the chat box, if you use the
/who command.
You've pretty much cornered everything else, so if he is NOT in the /who command, but he can STILL post, then there's certainly a bug.
There's the possibility that chat only checks against the fellowship roster when somebody first logs in. I'm merely fishing for a possible pattern, that you can help us validate.
If one of your other members is particularly adept at troubleshooting, they may be willing for you to temporarily expel them, for the purpose of testing exactly what's happening. That would be a very valuable contribution to our body of knowledge.