In previous versions (e.g., 1.20.4), creating a Superflat world with the "Water World" preset would result in normal spawning of aquatic mobs (squid, glow squid, fish) in the expected range (Y=50 to Y=63).
Since version 1.21.3, these aquatic mobs no longer appear at those levels. Instead, they only spawn at Y=-64, making it impossible to recreate the intended aquatic environment using the "Water World" preset.

Steps to Reproduce:

Launch Minecraft Java Edition 1.21.3 (vanilla, no mods or datapacks).
Create a new world, select "Superflat" as the world type.
Click "Presets," choose "Water World," and generate the world.
Check for the presence of aquatic mobs (squid, glow squid, fish) at their usual levels around Y=50 to Y=63.
Observe that no aquatic mobs spawn in the expected range. They only appear at the lowest layer (Y=-64).

Expected Result:
Aquatic mobs should spawn naturally within the water layers around Y=50 to Y=63, as they did in previous versions like 1.20.4.

Actual Result:
No aquatic mobs spawn at the expected heights. They only spawn at Y=-64, which deviates from the previous behavior of the "Water World" preset.

This issue affects all superflat worlds, not just the "Water World" preset, though the preset provides a convenient setup to test and confirm the problem.