Well...they may have been confused about whether that 5th payment was actuality made in year 2020 or in year 2021.
I don't know what else to say...other than maybe call MT again and see what another agent says, and specify again you made a 2020 Federal estimated payment in 2021...or seek a local MT tax preparer for their advice.
None of us helpers here actually lives in or prepares MT taxes (don't think so).....just interpret what we read.
____________*Answers are correct to the best of my knowledge when posted, but should not be considered to be legal or official tax advice.*
