There was a payment due which the client paid at the time of the return being e-filed. The letter from the IRS stated that payments are being received and posted to his 2019 tax year, and now they are waiting for him to send in a tax return.... The IRS has also been posting his estimated payments for 2021 to his 2019 tax year even though the client includes a payment voucher with each quarterly payment AND writes "2021 Est. Pymt" on each payment.

Apparently, the IRS is working through the pandemic "leftovers": I have had at least four clients in the past week receive letters that make no sense in reference to their 2019 taxes.