It might be that the IRS had in their records that you had received the $600 stimulus payment therefore they adjusted your refund.
If you received Notice 1444 in the mail but have not received your payment as mentioned in the notice, then you need to request a trace to be able to claim the Recovery Rebate Credit on your 2020 tax return. Please see How do I request a payment trace to track my first or second Economic Impact Payments?
The $600 stimulus was in January 2021, why would that impact my 2020 taxes? Different years, makes no sense.
The first and second stimulus payments had to be entered on your 2020 tax return to calculate the Recovery Rebate Credit correctly. The Recovery Rebate Credit is only for people who did not get the correct amount of first and second stimulus payment. If you received the first and second stimulus payment in full then you do not qualify for the Recovery Rebate Credit.