When you are late in your payments, the IRS will ask you to pay them interest. When they are late in their refunds to you, they will return the favor and pay you interest.
However, the catch is that this interest is fully taxable on your US Tax Return. To report interest paid from the IRS in MyExpatTaxes, please do the following:
Choose the Investment Option on https://app.myexpattaxes.com/wizard/grossincome
Choose the Interest Option on https://app.myexpattaxes.com/wizard/investment/
Add in your IRS interest on https://app.myexpattaxes.com/wizard/interest/2021