This is a bug in TurboTax that was reported in 2017 with regard to 2016 TurboTax but has not yet been addressed by Intuit. It also affects those who make an HSA Funding Distribution which similarly is not reported on Form 8606. I'll add your post to the list of those reporting this problem, but the list is small, so I suspect that this problem will remain low priority for the Product Quality folks.
I know of no workaround to prevent TurboTax from prompting you to provide an explanation for correcting your basis except to create a copy of the previous year's tax file, override on the IRA Information Worksheet the amount of basis being carried forward, then transfer in the modified file to begin the subsequent year's tax return.
