I had the same issue with my son born in 2021; didn't claim them on the advanced child tax credit. MFJ. We received 1800 out of the 3600 for our firstborn, and did not claim advanced for the one born in 2021. Do not believe the TurboTax software is currently working these numbers properly as there is no $3600 credit being given for the newborn, born in 2021.
