What I expected to happen was...:

When players are below Y=63 in superflat worlds, Phantoms cannot spawn.

What actually happened was...:

When players are below Y=63 but above Y=-63 in superflat worlds, Phantoms can spawn. Players need to stay below Y=-63 to stop Phantoms spawning.

Steps to Reproduce:

• Create a superflat world and stay below Y=63 but above Y=-63;
• Type the command below and Phantoms can spawn in night after 3 days.

  /tick sprint 6d

• Then stay below Y=-63 and type the command again, Phantoms cannot spawn.