On Desktop, no matter what I answer, it will not allow me to claim my wife's son on our joint return. If I change both of us to be his parents, it works. Is it a bug?

I am using Desktop Deluxe and it appears there's a bug in the issue as it pertains to children of only one of the 2 parents on a joint return.  It no longer allows me to claim my 17 year old even though he is living with us full-time and her ex is not claiming him.