I have this same issue. I'm amended my 2023 return because I filled out this section incorrectly, which carried over the incorrect basis in 2024 and 2025. Now that I have the issue corrected, Turbotax is not populating line 8 correctly even though all information is correct. I also deleted the form and re-entered it. Line 16 on Form 8606 shows "if you completed part I, enter the amount from line 8. Otherwise, enter the net amount you converted....." The converted amount does display correctly on line 16. Ideally, the two lines would display the same value for a backdoor conversion. I also previewed the tax forms with the worksheets and the Taxable IRA Distribution Worksheet is completed correctly - it's just not carrying over to form 8606.
