A CP12 Notice is sent when the IRS corrects one or more mistakes on your tax return which either result in a different refund amount, or in an overpayment when you thought you owed. From your notice and comments, it would appear you indicated on your filed return you did not receive your stimulus payments. This would have provided a recovery rebate tax credit for the amount of stimulus you indicated not received and increased your refund.
Since it appears the IRS is indicating your did receive your stimulus payments in 2020, you refund was reduced by the amount of the recovery rebate tax credit claimed on your return and lowering the amount of your refund.
