"Mob motion data" spawner parameter not working properly

The "mob motion data" values (X, Y, Z) for spawners are supposed to make the mob immobile unless it sees the player or something else hostile to it. The mob indeed doesn't move after being spawned, but closing the game and reloading the world causes the mobs to sometimes start walking around, after which they will be immobile again. This prevents map makers from positioning mobs in set places (that's what these data values are for, right?), and needs to be fixed.