In general i would say it's the key fobs. Pretty shure there are not a lot of people workshops that are able to decrypt an fsk signal and are able to identify what is wrong with it. The typical cheap key fob testers from china can't do that.
Unfortunately there are a lot of things that can go wrong with thes 9x6 key fobs. Why do i know? Because i offer to repair them:
https://986fans.de
Did the Porsche dealer try to learn a new key fob to the alarm control unit?