Claiming Additional child tax credit , married filing separate

Hi all. 
we live outside of USA. My wife currently has student loans ( on SAVE). She had foreign income less than $50,000. My income was $90,000+. We have 2 children. 

I filed married filing separate. I claimed the additional child tax credit under her return for $3400. I just got a letter from IRS saying they reduced her income and now not eligible for the ACTC. 

“

We changed the amount claimed as Child Tax Credit and/or additional Child Tax Credit on your tax return because there was an error on Schedule 8812, Credits for Qualifying Children and Other Dependents.

This error was in one of the following:

• Computation of the credit on
  Schedule 8812.” 

Should I amend my return and add the kids onto my return? In the past we’ve always filled jointly with no issues

thanks.