This is incorrect: "for software to not include first stimulus for which client was a dependent"

Perhaps it would help to review what is really happening:

The funds were paid out as Advanced payment against a projection. The projection used 2018 or 2019 tax returns. But 2020 is the Actuals. You use the 2020 return to reconcile what a person is entitled to, against what they got.

If the person is not a dependent in 2020, then they would be entitled to the payment/credit. That doesn't mean "not being claimed." It means "no longer qualifies as a dependent."