(All with version 19519):
Created a ship. Built a completely enclosed room around ship core in one session. During this session exiting ship core spawns player on top of ship core (and within the room) at least 5 times. Shut down game/server. Next session added blocks to increase size of ship, BUT did not change room enclosing ship core. During this and subsequent sessions exiting ship core spawns player outside/beneath ship.
This means that some other factor besides the space around the ship core affects where the player will spawn when exiting a core. This means the behaviour of this feature is not predictable.