The $6,000 was applied against the 2020 tax liability, increasing the 2020 refund or reducing the 2020 balance due, so the $6,000 has already been credited. It has no effect on any other tax returns.
Because $20,000 of the Coronavirus-Related Distribution goes on each of the 2020, 2021 and 2022 tax returns, that $20,000 has to be taken into account when figuring how much tax withholding and estimated tax payments need to be made each year. It's possible that since the $6,000 was entirely credited on the 2020 tax return that there was an underpayment of taxes for 2021 unless estimated tax payments for 2021 were made or tax withholding from other sources was increased.
