The state seeks the refund of aid for the time the NCP should have been paying it. Just because a NCP wasn’t ordered to pay support 15 years ago doesn’t mean the state can’t ask for reimbursement for amounts paid currently. For example, mother applies for TANF in Jan and starts receiving it. Lists NCP. State then send notices to NCP to pay, a process that might take several months. You bet that state is going to ask for the money from the beginning. If NCP says “Hey, it’s been 15 years and I didn’t even know this kid existed” it’s not going to change anything. If the mother has receiving funds for years and the father can’t be located, when they locate him they are going to ask for the money.
It is not an ex post facto law. The NCP always had the obligation to support children so there is no change to a law after the fact.