Perhaps it’s not knowing the law but I’ve ran into the trend (albeit small) where the NCP continues paying the CP support while having physical custody of the child.
In a typical example case, a CP mom sends son to go live with dad and dad agrees. Dad had been paying mom regularly and therefore owes her no back child support arrears. The mutual custody swap is done and son lives with dad for 6 months. All of this is done privately outside of attorneys, court, and the AG’s child support office. There’s no paper trail to prove the swap and dad (nor mom) follows up to stop, modify, pause/abate, etc. the child support monies still coming in through garnishment.