Thanks again for the help.
There is no "other income" on line 15.
The IRS form is CP22-A. It says "we made the changes you requested to your 2019 Form 1040 to adjust your pensions and annuities." I made no changes after filing in March. I called their phone (waited over an hour...ugh) and they told me even though I made no changes this is the appropriate letter because Roth conversions aren't subject to the standard deduction.
From what you're telling me, and the result of me doing fresh internet searches, I am strongly doubting that he is right, so it seems worth it for me to dispute it. He told me would give me two weeks to do research. I ordered an IRS transcript today. Unfortunately I couldn't get the online to work, so it will be 5-10 days to deliver it.
My plan is to dispute it. Hopefully I get the transcript within the 2 week window and can then call them to tell them I'm disputing it, armed with better information about what they flagged. There seems to be no other way to indicate my wanting to dispute to them. If there is please let me know (and save me another hour in the phone queue).
