Same thing happened to me. I mailed my 2019 tax return by certified mail on 7/13/2020 along with my check for $2,955.00. The IRS cashed my check and the USPS tracking shows the IRS received my return on 7/21/2020. I just today discovered this while trying to E-file my 2020 tax return (I had to file an extension for 2020 so that's why I am late in filing). My 2020 Federal E-Filing was rejected due to not being able to match the 2019 AGI amount. I changed the AGI to zero and then the E-filing was accepted. Now I guess I have to mail my 2019 tax return again. I wonder if the IRS will be able to match the return with the check I previously sent? I hope my funds didn't go into some lost & found account rather than my account.
