In some cases, it seems like the IRS (representative) disagrees with the 1040 pg2, line 28, refundable child tax credit. In 2021, the IRS gave out advanced payments of the CTC totaling half of what the credit would normally be at tax time. For example, if you have 2 dependents under age 6 and your credit is $7,200 then you would have received $3,600 during the year and $3,600 as a credit on your return. The advanced payments are reconciled on form 8812, and any remainder flows to line 28 of the 1040. Review form 8812 if line 28 is in question.
The IRS sends letters and notices of proposed changes. I encourage everyone to challenge those letters for accuracy.