Dumb laws like this end up rewarding folks who waited until the last minute to file their returns.  Not something I think should be rewarded, but that's how the tax law was written.

You can literally take two people with identical financial situations.  Let's say both single with $70K of AGI in 2019 and $85K of AGI in 2020 and 2021.  Person 1 files their 2020 return in early February 2021 when efile opens.  Person 2 files an extension and files their 2020 return in mid-October 2021.  Person 1 is not eligible for EIP3 because they have a 2020 return on file with too much AGI.  Person 2 gets $1,400 because it's based on 2019.  Procrastinators win!

I had a stack of 2020 returns that were EIP3 eligible based on 2019 but over the limit based on 2020 income.  They were held until October just in case there were any EIP4, 5, 6, etc. laws passed.  I had a couple folks who wanted to go ahead and file early anyway but the majority of people who were in this situation (myself included) agreed to wait.  Turned out it wouldn't have mattered but nobody knew that at the time.

I had at least one client burned because we filed his 2019 return "too early" and he missed out on EIP1 (or maybe it was EIP2?)  He was pretty grumpy about it but I pointed out the "no stockpiling" rule where I'm required to file within 3 days of him giving me the signed 8879 (and it's not my fault Congress writes dumb laws).  Sorry, my crystal ball didn't tell me that Congress was going to write a law that retroactively penalized you $1,200 for filing your tax return "too early" in the season.

Rick