"but shouldn’t the Net PTC take into account the amount of advanced subsidy received in order to prevent double dipping?"

It's the other way around: The State PAS takes the PTC (which your client didn't get in advance) into consideration, but the Fed 1040 credit doesn't take the PAS into consideration. Your client had no APTC, so there is only the CA PAS amount, which is the net subsidy after the federal PTC is taken into account. Does your form show they have to pay back the CA excess APAS?

I found you a similar discussion topic, but note it is from before the ARP Act:

https://www.caltax.com/forums/topic/form-3849-refunding-federal-premium-tax-credit/