This is a bug in TurboTax that was introduced with the first release of 2020 TurboTax over a year ago. Your only option at this time with TurboTax is to use the CD/download version of TurboTax, override Form 8606 line 20 and enter the correct amount of earnings distributed from the Roth IRA to which you are applying to the first-time home purchase. With the override you'll need to print and mail your tax return.
