There apparently have been frequent updates to the Online product (which, because it's Online, can happen at any time without notice). I say this your description of the issue does not make sense on its face (ChatGPT notwithstanding).
My guess is that TurboTax was thinking that you did not have HDHP coverage in 2025 (which is correct), and that you did not have HDHP coverage in every month of 2024 (which you said is not correct). This would have triggered the last month rule and the penalty.
If you are using the Online product, let me suggest an HSA Reset, which is a process to remove all of your HSA data (because I think that TurboTax got confused) so you can start over.
And since I believe that the Online product has been updated to remove the possible issue of TurboTax conflating your data, your issue should disappear.
Just remove all HSA data as shown below.
HSA RESET
***Reset***
1. make a copy of your W-2(s) (if you don't have the paper copies)
2. delete your W-2(s) (use the garbage can icon next to the W-2(s) on the Income screen)
*** Desktop/Windows***
3. go to View (at the top), choose Forms, and
4. select the desired form (1099-SA (if one), 8889-T, and 8889-S (if one)). Note the Delete Form button at the bottom of the screen.
*** Online ***
3. go to Tax Tools (on the left), and navigate to Tools->Delete a form
4. delete form(s) 1099-SA (if one), 8889-T, and 8889-S (if one)
5. go back and re-add your W-2(s), preferably adding them manually
6. go back and redo the entire HSA interview....
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