ok I see the drill down for that worksheet around line 15/16 and states it's for 'contributions in 2024 that may be partly nondeductible'
Never encountered this worksheet I don't think it is normally used, and I'm not sure what situation is meant to trigger this, it says line 15 would have an asterisk in it. From what you described your 2024 contribution was fully non-deductible and a pretty routine situation which I don't think should be hitting this worksheet.
Do you have anything else going on, was part of the contribution set up as deductible, any other distributions, is line 15 on your 8606 showing some sort of exception?
If this is routine I still think you have hit some sort of data issue or inconsistency in the interviews, you can just delete 8606 and go back thru the interviews for IRA and 1099-R to see if it cleans up.
Also where did you land on the original question for line 6 balance as of 12/31/24, I don't think this should include the contribution made for 2024 in 2025 contrary to some of the guidance being provided above - Form 8606 instructions "Although the contributions to traditional IRAs for 2024 that you made from January 1, 2025, through April 15, 2025, can be treated as nondeductible, they aren’t included in figuring the nontaxable part of any distributions you received in 2024."... since the 2025 contribution is not part of your basis in line 5 when it is divided by the total value in lines 6+7+8 on line 10, it shouldn't be in line 6 either. The tax on your Roth conversion is figured by where you stand as of 12/31/24, if you happen to make a contribution for 2024 afterwards in 2025 that it just adds to your basis going fwd but has nothing to do with the Roth conversion.
