The SEP IRA custodian only records and reports the year in which a SEP contribution is made, not the year for which the contribution was made. It's up to you to allocate SEP contributions made between January 1, 2021 and the due date of your 2020 tax return to either 2020 or 2021. Still, you would probably want to make sure that whatever portion of the April 2021 SEP contribution you allocate to 2021 is supported by your 2021 earnings up through the date of the contribution.
