We have the same issue...this is what I just found:

From IRS website: https://www.irs.gov/newsroom/irs-operations-during-covid-19-mission-critical-functions-continue

Pending Check Payments and Payment Notices: If a taxpayer mailed a check (either with or without a tax return), it may still be unopened in the backlog of mail the IRS is processing due to COVID-19. Any payments will be posted as the date we received them rather than the date the agency processed them. To avoid penalties and interest, taxpayers should not cancel their checks and should ensure funds continue to be available so the IRS can process them.

To provide fair and equitable treatment, the IRS is providing relief from bad check penalties for dishonored checks the agency received between March 1 and July 15 due to delays in this IRS processing. However, interest and penalties may still apply.

Due to high call volumes, the IRS suggests waiting to contact the agency about any unprocessed paper payments still pending. See www.irs.gov/payments for options to make payments other than by mail.