I don't have experience with this specific use case.

I don't understand why Turkish Airlines is making the eligibility determination. There is no such thing as a *A flight. Only *A partners. And it is up to the operating airline to provide the credit to the receiving program.

I wonder if the flight is showing as AIX or Vistara (not *A partner) flight in their screens because of the prefix and a database that has not been updated.

It is worth asking TK as to the reason in specificity than their stated one liner that it is not a *A flight. That will determine your next steps.

Here is the Turkish Airlines earning page for AI flights.

https://www.turkishairlines.com/en-i...nes/air-india/

Your boarding pass with Air India (not express) should be sufficient to appeal. If the boarding pass has a *A logo then even better.