Maybe Cora had him imprisoned somewhere magically, and when she left, he was able to get out?
Maybe his old buddy Merlin also managed to ward off the Curse (or was just inside Cora’s bubble) and is still there and found him and they’ve been shacked up together?
If so, maybe Merlin will be the one to heal Nealfire? Maybe he even helped with Phillip’s soul? (I know we’ve pretty much assumed they’ll just use Lake Nostos for both of those, but just throwing the alternative out there.)
If they do bring him back in present day, I’ll totally be shipping him with Mulan.