The possible reasons: SSN incomplete, Last name discrepancy, adjusted gross income exceeds $150k (married joint file_ or amount computed incorrectly.
If the IRS reduces your refund, they will send you a letter explaining why it was reduced. You can respond to their letter and explain why the refund should not be reduced and they may reverse their decision if you have a good reason.
That is usually the list of generic reasons they give if they changed the Recovery rebate credit on line 30. How much is on 1040 line 30?
Do you have an amount on 1040 line 30 for the Recovery rebate credit? Did you not get the 3rd Stimulus payment? Or qualify for more? Line 30 is only if you qualify for more or missed getting one.
If you claimed a missing stimulus payment on your return but the IRS took it off you have to ask the IRS. They think they already sent it to you. Maybe it went to an account you don't remember. Or you got a check or a card. It was probably easy to miss the debit card in the mail and think it was junk mail. They have to put a trace on it.
Check your IRS account to see what they sent you (check both spouse's accounts if married)
Your Online Account | Internal Revenue Service
Request a trace for a missing third Stimulus payment