I'm pretty sure this is a thing. It just doesn't tell you. I've joined servers before which were full and my game doesn't tell me it's full, it just adds me when there's space.
Maybe someone can verify this because I never heard of it before?
Queuing people up to join rooms when a space is available seems abusable - consider someone attempting to dos by flooding a room / rooms with connection attempts. Rather than being blocked out, the server now has to add them to an invisible queue, taking up time and memory which is exactly what the dosser is after.