I efiled a 2021 return in 2022 that I have the Submission ID for and my client received a letter from the IRS last year stating they did not file 2021. We had to send in a paper copy.

I now have a different client that I efiled his 2022 return in 2023 that I also have the Submission ID for that just received a letter from the IRS stating that they did not file 2022. 

In both cases we checked their IRS accounts and sure enough it showed that they did not file.

Are other people experiencing this problem? How is this happening?