I worked for an insurance comapny for 13 years. The pre-auth in no way authorizes approval of the payment for the service, just that the actual service - if meeting all other requirements (ex: medical necessity in this case) is able to be considered. They gave the doctor a disclaimer when they called for pre-auth - I have it memorized in my head & haven't even worked for the insurance company for over 10 years: "This authorization in no way guarantees payment of benefits. Benefits are payable upon satisfactory receipt of proof of loss & in accordance with the certificate of insurance."
It's frustrating - many doctors offices don't care enough to let you know - most people assume the pre-auth means pre-auth of payment. It actually happened to us last year with $400 of blood work for my son. We had the pre-auth but it was denied as "experimental, not medically necessary." I got all his medical records & a note from the doctor to send in, and they reversed the decision & paid.
OP get the doctor to write a note stating the diagnosis & why it was medically necessary and appeal the claim. That should be all it takes to get this cleared up. Good luck - I know how frustrating this is!