[MC-277216] Players need to stay below Y=-63 instead of Y=63 to stop Phantoms spawning in superflat worlds

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
```

Attachments

Can confirm.

In your video you are at Y=-60 which is still the overworld (-64,320). If you go below Y=-64 do they still spawn?