Yes, I own the Roth IRA annuity. The non-spousal inherited IRA is a traditional IRA.
The annuity is variable, and the payment varies from month-to-month. The Nov. 2020 letter about the calculation error just gave the amount of the total underpayment (covering all months of underpayment) and showed what the Oct. 1 2020 payment would have been without the calculation error. There were no further details. From the Oct. 1 underpayment I have figured out that the annuity was underpaid over about 30 months.
The November letter said “Please be aware that the amount will be taxed as income . . .” I should have questioned that back then.
Yes, I agree the withholding is a minor issue.
Although line 4 of Form 4852 says “I have notified the IRS of this fact”, the instructions just say “ . . . you may call the IRS . . . for assistance.” I’ll give them a call, and if it turns out to be an impossible wait, I’ll just note on the Form 4852 that I tried to reach them.
I also posted this issue to a TIAA discussion group on community.morningstar.com.