It seems the IRS had records that showed you received the stimulus payments.
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?
There has been a lot of confusion with the recovery rebate credit. When you prepared your 2020 return did you enter the amount that you had already received? Did you receive $2400 last year when the first stimulus checks went out? The IRS cross checks for stimulus money that you already got, and if you said on your return that you did not get it, they take it away from your refund.