I would not amend your 2020 tax return solely to get the 3rd stimulus payment. But if ''unchecking'' the box on your 2020 tax return creates an additional refund, you should file it - those can be e-filed this year which should help. But you may want to delay filing it since the IRS is currently in such disarray and you have 3 years from the date you filed your return (or 2 years after you paid the tax due, whichever is later) to file an amended return.
The recent guidance NOT to amend was mainly for the unemployment income exclusion that was just recently enacted. They made yet another change to that just yesterday, so they are urging taxpayers NOT to amend to exclude it. You can read about that here. @harryhoudini66 A post that references the changes is here.
On your 2020 tax return, how did you file? How much stimulus did you receive and how much recovery rebate was claimed on Line 30? And are you saying that the IRS issued you a refund, but without the amount on Line 30? If so, did you receive a notice in the mail?
Although you can e-file amended 2020 tax returns starting tomorrow (1040x is tentatively scheduled for 03/26), you may want to delay doing that until further guidance is released by the IRS.
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