I assigned a scoreboard value to an entity, then saved, quit and reopened the world. Upon reentry the entities UUID had changed and the scoreboard values were assigned to a no longer existant entity. Even creating an entity with a custom UUID did not preserve it's value. I have tested this with multiple types of entities to the same result. To replicate this effect, generate any entity, find it's UUID, save and quit, and go back into the world. The entity will no longer have the same UUID.