I had a recent experience where a newly set up user merged with an old exited user - MS advised that this was because it had the same primary email address as the previous user; the 'fix' is to create a new user with different upn and email address; I have also noted whilst looking at this issue that CRM seems very random about updating feilds (still had old position description, which couldn't be updated because it was synced with AAD - except it is stale information, and does not update on refresh of user, sometimes change of email address will sync, sometimes not. Some way of unlinking synced users, or at least having reliable sync of attributes would be good.