In my game the difference is also 30 XP. Meanwhile , we met Astarion and he came in with the same XP my player has. (So also 30 above Shadowheart)
I experienced the same pattern. When I further added Gale he also, like Astarion, had the same XP. Furthermore, it has continued. When my character reached level 3, with Gale and Astarion, Shadowheart was delayed by 30xp or so.
