This is so strange. If Affirm is the creditor why does he not ask Affirm to initiate a chargeback against the merchant? Is this one way Affirm can compete against credit cards, that they do not need to handle this kind of situation?
Also, after reading this account I would be very hesitant to go through Affirm for anything. Chase seems responsive on the other hand.
For direct merchant/service provider purchases. Different beast than an installment loan, legally speaking. Regardless of Affirm botching things after the loan was paid, I suspect that the party reversing and withholding payment on the loan was taking a contractually and legally indefensible position.
This write up looks like a great argument to just try and work through the system first.