Ocean mobs do not spawn naturally or through Spawners/Trial Spawners on Superflat worlds

Ocean mobs do not generate in superflat worlds, not naturally, not through any kind of spawners. Tested with Nautilus and Pufferfish.

Steps to reproduce

[Natural]
1. Generate superflat world with preset Water World

2. Navigate through the ocean, check all the way to the bottom.

Expected: Passive mobs spawn in the ocean.

Actual result: No mobs are naturally spawned besides those generated by structures.

[Spawner] - Test with Nautilus

1. Generate superflat world

2. Create 5x5x5 water box within y=38 and y=58

3. Place a spawner within the water box leaving at least 1 block of water on top and 2 blocks of water below the spawner

4. Set the spawner mob type to Nautilus using a Nautilus Egg.

Expected: Nautilus is generated by spawner.

Actual result: No mob is generated.

Probably related to MC-278019