Some further info here. It seems that the 2021 Recovery Rebate is indeed intentionally based on 2021 AGI. This is because the third stimulus payment was considered an advance payment for 2021 taxes. While this makes sense, it defies some logic because in reality the payments were 100% based on 2019 or 2020 tax returns when issued. So it seems that basically if you're in the unusual situation I find myself in, there is no recourse because the recovery rebate is based on the principal that you situation got worse in 2021 and hence you should get more stimulus, not that the government needs to count your amended 2020 return but IGNORE your 2021 return. Seems a little ridiculous that because I filed an amended return 12 days after receiving the third stimulus payment, I have no way to have the IRS use that amended AGI to recalculate the thing they calculated two weeks earlier.
