Sorry that's the way it works out. Your stimulus check is based on the number of qualifying people listed on your tax return for 2020. You don't receive a check for the new baby necessarily, it is just based on the number of qualifying people listed on your return.
So, if you lost one qualifying person because they turned 17 or you aren't claiming them this year and replaced that person with a new baby, you still have the same number of qualifying persons on your return.
