I have a little bit more to report on this. I made another call to TIAA, and got a slightly more meaningful explanation, which indicates it’s not just a coding error:
TIAA at the outset, before notifying annuitants of the calculation error and the correction, decided they would report the amount of the “remediation” as taxable. “Remediation” is the word used by the consultant I talked to. I wonder if that is significant.
Up to now I had thought that probably the calculation error had occurred with all annuity payments, regardless of source, and that TIAA had slipped up in not tailoring the correction procedure to what is probably a very small percentage of Roth annuities.
But apparently they did this on purpose.