I believe I've found the confusion. My wife says she thought we got a stimulus check in 2020. I've gone through our records for 2020 (which I hadn't done before, because I thought all stimulus checks came in 2021). We got a "CARES Stimulus" check in June 2020. I believe that was our first stimulus check. So the $1200 check was really our second check, and we really did get a third stimulus check for $8400.
I hope that someone else will benefit from my struggle in this. Thank you to those who've helped me with this confusing issue.
