I don't agree with your response. I have EXACTLY the same situation as lhopkins25. My spouse had an HDHP for all of 2024 and the first 9 months of 2025. Then had medicare the last three months of 2025. Since they had an HDHP and HSA for all of 2024 and previous years, I do not believe the "last month rule" applies.
Turbo tax asks for the kind of HDHP coverage for each month of the prior year - 2024. I filled in self plan for each month and Turbo Tax computes income of $429 plus 10% penalty under the last month rule.
If I go back and change the Jan 2024 month to say they had "Medicare or None", then suddenly the additional income and penalty goes away. So something is definitely wrong in TurboTax.
