The CTC & ACTC are not separate things ... the ACTC is a by product of the unused portion of the CTC and computed automatically by the TT program and the IRS so if the income changes the entire return is recalculated from that point on ... it is like a river where everything flows downstream. It is highly unlikely that the IRS would get this incorrect since the one is really a part of the other and the computers rarely make these kind of "math" errors but it is not impossible especially this year.
If you use the TT amendment program to recalculate the return for yourself you should get the same end result as the IRS does ... if you don't then mailing in an amended return is the course of action you need to take HOWEVER the column A of the amended return must start with the changed one the IRS did for you which may not be easy if you say they missed the ACTC ... if this is the case you will need to use the downloaded amended program option and make the changes in the FORMS mode.
