Per the IRS Instructions for Form 1040-X webpage:
If the IRS doesn’t use your overpayment to pay past due federal or state debts, the refund amount on line 22 will be sent separately from any refund you claimed on your original return (see the instructions for line 18). We will figure any interest and include it in your refund.
Note that any link in the information above is updated each year automatically and will take you to the most recent version of the webpage or document at the time it is accessed.