Can't spawn passive creatures in my custom biome

This has recently been making me annoyed. For some stupid reason when I had a entity thats a creature spawn, it just wont spawn. It doesn't want to. I even used vanilla grass as the ground block and recolored it, and it STILL does not want to spawn.

If you have any solutions please reply.

And if this is just a bug, MCreator please fix this if you could.

Thanks.

More details:

MCreator version: 2023.3

Mobs can still spawn, it's just the creatures that refuse to spawn.