Hi, I am a VITA "tax counselor." I have the same certifications as you. This is my fifth year in about 15 years doing this. Many at this forum are way more expert than I am.
For now, things to consider (if you have not already):
-- The IRS is supposed to receive all 1099-Divs, original and corrected, without the taxpayer having to submit them.
-- Do have the taxpayer get a copy of his "transcript" as described here:
https://www.irs.gov/individuals/get-transcript . Some taxpayers find this laborious, but the transcript is often an invaluable tool for sleuthing.
-- The IRS bills interest. See for example
https://www.irs.gov/newsroom/interest-rates-increase-for-the-first-quarter-of-2023. Have you checked for how much this interest is?
-- Many feel amending with Taxslayer is tricky. To try to get to the bottom of this, I would use a phony name in the 2021 Taxslayer software, training version or non-training version, and carefully work the 2021 taxes again, using the corrected 1099-Div.
Then compare the originally submitted 2021 Form 1040 (and related forms) to the 1040 and related forms that the scratch file spews out. Suss out the differences. Compare to what the IRS say. And so on.
-- Once you feel certain that the IRS is mistaken, using the Taxpayer Advocate
https://www.irs.gov/taxpayer-advocate is fine and may get results. But also consider contacting your state's U. S. Senators. Where I am, and regardless of one's political
party, both Senators' staffs are incredibly effective at getting results in a timely fashion.
--
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