I don't see this as a valid point. It was income, that you chose to defer. How was it reported? Certainly not in a way that cost you any taxes.
Contributions to tIRAs and 401Ks are not yours to spend until you pay the taxes. So now you are converting and reporting the income, which means you pay the taxes now, which could include IRMAA. If your spreadsheet doesn't count conversions as income that's a pretty basic error that's on you.
First I agree on your last point. My spreadsheet was wrong. That is on me.
As far as where was it “reported” years earlier?
1) In the case of employment income, employee’s gross income is reported on the annual W2 IRS form.
2) FICA withholding is calculated on one’s “gross income”, not net after 401K deductions,
3) FICA includes both SS and Medicare contributions
4) IRA deduction to gross income is done (today) on Schedule 1 as an adjustment to income on tax form 1040.
So both IRA’s and 401Ks were reported (albeit, not taxed) in the year contributions were made.
BTW, I didn’t chose to "defer the income" back then, I chose to defer the tax paid on that income in exchange of having limited access to that as retirement savings.
Considering #2 & #3 above, I have already paid the FICA tax (includes the Medicare tax) on all 401K contributions when they were made way-back-then as reported on my W2 years ago. As such, IMO, it should be excluded from any of today’s income based additions to Medicare. A point could be made about a Roth conversion from an IRA being treated differently because there was no FICA paid on those IRA contributions. However any rollover from a 401K to an IRA before today would mix those dollars making treatment today as being difficult at best.
To be clear, as I am paying the conversion tax from IRA monies, I have no issue about including that tax as income in the year of the conversion and thus subject to IRMAA. It wasn’t “converted” to the Roth. I do think that IRMAA should not be based on the total Roth conversion amount.