We are running a very recent build (not on current or monthly channel) of Office 2016 / SfB 2016 ProPlus and have had two reports of this strange (and embarassing) behavior. Occasionally, when you click on a contact to start a conversation it will bring in a random person from your contact list into a multiparty chat. You can close the chat window and reclick the same single contact and it will bring that same random user back into the session. The random person has never been used in a previous multiparty session with the first user so it's not like it is a reused session. Sometimes the issue goes a way on it's own after a few minutes or when you restart the client. You can imagine how this issue can result in some unwanted sharing of information so we are really concerned and would like to know if others have seen and corrected this behavior.