Receive prior authorization--now being charged?

They can if you allow it. Do you have a copy of your insurance plan? If so read it and check the exclusions. Did you ask what stimulated the delayed billing other than "additional research" and what prompted that inquiry. After you do some of your own research, find a lawyer that deals with medical insurance issues and get an opinion. And why are you no longer covered? Who and why was that decision made. Additionally, ask your lawyer why it was approved initially and why did they then disallow the service.