I was using mongo:latest in Docker setup and auth did not work at all. Then I found this Reddit thread and I tested versions 3.6, 4.0, 5.0, 6.0 and 7.0 from which after 5.0 auth no longer worked.
Perhaps something has changed in 6.0 with auth. 5.0 is better than nothing, but it'd be very nice if latest could be supported.