I agree with Mike, don't appease. We either had that nonsensical answer or I read about it here.
Honestly if the letter is what I think it is there is a place to indicate that the return was in fact filed but with a different TIN; complete that section and it should go away.
I have pushed back against this, asking them to focus on 'intent' fwiw of their deposits and that while a cpa may thrive on creative accounting, their tpa cannot defend this approach.
Tommy - FWIW, I don't think that 12 providers/vendors/platforms or whatever are necessarily too many. We are just a fee-for-service TPA, and many of our referrals come from CPA's and investment Advisors. They, and their clients, choose the investments and the platform. So we work with a lot of platforms/providers. Mind you, it would be NICE if they'd work with a limited number, but we have no control...