The $1200 reduction may represent a disallowed Recovery Rebate Credit. Any first and second Economic Impact Payments (stimulus payments) that were issued to you reduce the amount of any 2020 Recovery Rebate Credit that you claimed. Call the number on the CP12 notice for more information.
Understanding Your CP12 Notice | Internal Revenue Service (irs.gov)
2020 Recovery Rebate Credit | Internal Revenue Service (irs.gov)
