The bottom line is that the results you indicated seeing on your Form 8606 and the worksheet are as should be expected for the transactions that you performed.
Normally the basis from the $4,800 nondeductible contribution made in 2020 for 2019 would not apply to the 2019 conversion, but the use of the worksheet changes that. The worksheet does apply the $4,800 of basis to the 2019 conversion. TurboTax is rather liberal in its use of the worksheet and probably should not be using it in cases where the deductibility of the traditional IRA contribution is known without having to consider AGI, such as when you explicitly choose to make the contribution nondeductible or your modified AGI would be above the threshold even if reduced by the entire amount of the traditional IRA contribution, but TurboTax uses it anyway.
