If you take an RMD or a plain distribution both are taxable. Even if you say it wasn't a RMD it will still be taxable. For 2020 nobody needs to know it was the RMD. There is no RMD Requirement. So it's really not an RMD in the first place.
I assume if you put back a distribution they will send a 5498 to the IRS the same as if you rolled over a distribution to another account or made a new contribution.
https://www.irs.gov/pub/irs-pdf/f5498_20.pdf
