The basis added with a 2018 contribution could have been entirely applied to a 2017 distribution or conversion if 2018 TurboTax used Worksheet 1-1 from IRS Pub 590-B, leaving 2018 Form 8606 line 14 iwth $0 as long as the 2017 year-end balance was zero. The result in 2018 TurboTax seems to indicate that 2018 TurboTax is not using Worksheet 1-1 but is instead doing the entire calculation directly on Form 8606.
(I haven't figured out a good way to predict whether or not TurboTax will use Worksheet 1-1. There are cases where Worksheet 1-1 must be used, but it seems to me that TurboTax sometimes used Worksheet 1-1 in cases where it is inappropriate to use it, cases where it's clear to me that there is no potential for deducting the IRA contribution.)
