I do believe it's an EA thing. Shadowheart might be important to the story because of the box, but I think if she leaves us the story will conveniently put her near us, but not back at our camp... and probably make us face her if we already have the box and she left for some reason.
Also EA is riddled with strange things as us carrying Astarions corpse around as the camp moves, or him standing there still in his undies when told to leave.
