Did the expert ask you how much you got for the first 2 stimulus payments (which included any received in 2021 for the second payment) ???
If they did and you gave them the incorrect answer then they entered the wrong amount on the return which populated a credit on line 30 of the form 1040 for the missing advance payment. This was not an input error by them it was a misunderstanding on your part for assuming the payment you got in 2021 was not what they were asking about.
Thus you did get the stimulus money so you don't also get the credit on the 2020 return ... that is the simple fact so the IRS recalculated the return to remove the credit you can not have again.
