I can think of two issues that can cause this:
Session
Auth::user();
accesses the user session in order to get the token from which it can identify the user in the model, so flush the session and try to login with the other users, using:
Session::flush();
Database
I would want to go to the database to make sure everything is under the logic expectations, take a look at a couple of rows of the user table, especially the ID column and the Email.
i don't think that this should be considered as a direct answer, rather than where to look and what might cause the problem, hope it works out with you!