Wow. This is the exact problem that I had this year too.
I also made a mistake on my 2023 Form 8086. Line 4 was zero when it should have been $8000. I forgot to enter my contribution for 2023 that was made between 1/1/2024 and 4/15/2024.
So I adjusted my basis for my traditional IRA in my 2024 return and added an explanation that this was due to an error in 2023 Form 8086. In the Turbotax Home & Business edition that I'm using, when you see the prompt asking you about the basis for your traditional IRA, there is an "Easy Guide to determine your basis" -- click on this and then make the adjustment. Once TT sees the adjustment, it will prompt you to "prepare and explanation" for the adjustment.
Once I entered my new basis of $8000 (instead of zero), Line 4b went to zero and my Backdoor Roth conversion was no longer considered "taxable". Finally, done!!!
Took a long time to find this thread and it was spot on. Thank you!
