Are you saying "mixed" is just one racial classification? Or do different mixtures of the "3 pure races" give rise to different races?
No, but they change the list of characteristics you'd use to identify that group. At least, that's how it'd work if you weren't practising pseudoscience.
What if said "anomaly" has a child with another similar anomaly? And their offspring all do the same? Do they get their own racial category that's independent of the original group?
Sounds like a social construct to me. Also, no, there are arguments in Africa about racial groupings, too. It can depend on the question being asked, and to whom the question is asked. Black isn't a race if it can be worn by anyone with a non-Black parent, surely.