In determining for which taxable year a designated nondeductible contribution is made, the rule of section 219(f)(3) shall apply.
219(f)(3) is the provision to apply a contribution to the previous tax year of made before the due date of the return.
I see nothing that says that the 590B worksheet 1-1 is not correct and the intent of the law is to allow retroactively applying the non-deductible contribution contributed *in* 2020 *for* 2019 to 2019 for the purpose of calculating 2019 distributions of that contribution.
(The section of the code that deals with the taxation of distributions does not seem to address this at all.)
That might be why many tax software programs started using the worksheet instead of 8606 for doing all the calculations.
The language of the actual law is quite different than the language used by the writers of the 8606 and 590B.
