The bug

In the provided data pack, two dimensions are added with the y axis bounded between two heights either below y=40 or above it. It seems that slime spawning is completely fixed to y=40 somewhere in the game's code, completely forbidding them from spawning in dimensions which do not allow blocks to exist at that point or below.

I have not tested this for ocelots, squid and other mobs with fixed y-axis bounds for spawning but they may also be affected.

To reproduce:

1. Create a new world with the provided data pack
2. Teleport to the "deep" dimension
   → note the high amount of slimes spawned
3. Teleport to the "sky" dimension
   → note the complete absence of slimes

This could be fixed by having these mobs with such y-axis bounds be automatically scaled based on the dimension's height values, or even better, be made independently configurable themselves as a separate setting in the dimension specification.