I had three trips to SJU this past month. On each of them they offered sparkling wine during pre-departure beverage on the flight from ATL and JFK to SJU. No sparkling wine was available on any of the returns on ATL or JFK. It seems they only cater a bottle or two in ATL and JFK but do not restock it in SJU.

So if you want to have the sparkling wine, get it pre-departure on your way to SJU.