No, this is not a software error. Both the Credits For Contributions to QCOs and QFCOs (Form 321) and the Credit for Contributions Arizona Form to Private School Tuition Organizations (Form 323) operate in the same way: if you take what was a charitable contribution on the federal return as a tax credit on the state return, then you have to subtract the amount from itemized deductions on the state return.
The problem is that the state instructions refer only to form 321 and not form 323, but the line 6C that you refer to says, "Total charitable contributions made in 2020 for which you are claiming a credit under Arizona law for the current (2020) or prior (2019) tax year".
Both of those credits are covered by this description (and possible some others).
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