BACKDOOR ROLLOVER

First of all, it's either a Backdoor (which is a conversion process) or it is a Rollover.

The word Rollover is used to describe a movement of funds from one retirement account type to one of that similar account type.

A Backdoor Roth is a conversion. It relies on the funds in the pre-tax account to not have been pre-tax, to have been Basis already, so it moves post-tax funds from a tax deferred account to a tax exempt account. There would be no tax owed on the conversion, when the only funds that exist are already taxed (basis).

A 1099-R is issued for the year the distribution happened. Not the "year of funds." It's the reporting of the event. If they converted in 2022, that will not be on any tax form yet; they will get a 1099-R for 2022 and it goes on the 2022 tax filing.

You did not describe when the 2020 funds were deposited to the Traditional IRA or when the 2021 funds were contributed to the Traditional IRA. Both contributions would need to have been non-deductible. If both were made at the same time and then immediately converted, and you have a 2022 conversion, that doesn't make sense for the 2020 amount, because it could not have been just recently contributed.

The Backdoor functions as non-taxable because:

Everything in the account is Basis and only Basis; it is moved (converted) so quickly that there are no earnings, so there is nothing taxable at all . Any other scenario is going to mean the conversion is taxable on a pro rata basis.

Hope that helps.