I found something that worked for me…“Keep Trahearne alive” I encountered the same but when i repeated the quest twice letting him die(Trehearne) and the other is keeping Trahearne alive at all cost made it. (Just to make sure if it will trigger the next scene). It seems that when Trahearne died and revived, that’s where things get bugged. he won’t follow you anymore upon the Souther exit stage thus staying outside the cave. Hope it helped.
IGN : Venice (Isle of Janthir)