What I expected to happen was...:

When players are under sea level in any Superflat world, Phantom cannot spawn.

What actually happened was...:

When players are under sea level in any Superflat world, Phantom can spawn.

Steps to Reproduce:

1. Create a Superflat world and stay under sea level;
2. Type command under and Phantom can spawn in night after 3d.

   /tick sprint 6d