I think I overlooked the fact that you made a $5,500 nondeductible traditional IRA contribution in 2018 *and* an earlier nondeductible traditional IRA contribution in 2017. If that contribution in 2017 was *for* 2017, that means that your 2017 Form 8606 should have shown $5,500 on line 14 which transfers to 2018 Form 8606 line 2 to be combined with your 2018 contribution on line 1 for a total of $11,000 on line 3. This would indeed make your $5,500 Roth conversion in 2018 fully nontaxable, leaving you with $5,500 basis in your traditional IRAs (2018 Form 8606 line 14). Check your 2018 Form 8606 to see if the amounts I mentioned are present.
