I understand your discomfort. With so many different types of MAGI, it is hard to make decisions.
My experience in this area resulted from appealing to a lower IRMAA due to a life-changing event. You can find the rules at this link, and the information at this link more clearly defines the rules. https://www.ssa.gov/forms/ssa-44.pdf
Please look on page 6 of 8 in the first paragraph of Step 2. There it clearly states, "Your MAGI is your adjusted gross income as used on line 37 of form 1040 plus your tax-exempt interest income as used on line 8b of IRS form 1040." The line numbers refer to forms prior to TY2018, but the method appears to be clear.
Also note Step 3, on page 2 of 8, of ssa-44 for entering the data that defines MAGI for the purpose of IRMAA. It only uses AGI and Tax-Exempt Interest income. The terms AGI and Tax-Exempt Interest Income repeat the definitions located near the bottom of Page 6. And just in case we didn't get the messages, SSA again defines the definitions for Step 3 in the Instruction section on Page 7 of 8. So I think the definitions in SSA-44 logically indicate that the untaxed portion of Social Security is not added into that MAGI. I'm not aware of any changes to the rules, but please understand that I can't know what I don't know. I share your discomfort becaues an error comes with a loss of money.
