I have the same issue! I mailed our return and check in July of 2020 and just recently found that they have no recored of the return but have cashed the check. Have you found a good answer to your question??? Should we send another copy of the 2019 return along with a picture of the cashed check?? It is next to impossible to get answers on this issue or get ahold of the IRS.
I haven't done our 2020 taxes yet, I appreciate the advice about the stimulus $ on that return.
