Small objects not spawning in custom biome

I’ve recently found a bug in a couple of my biome mods: small objects (anything except trees) aren’t spawning. This can be observed very clearly in my WIP jungle biome:

I’m not going to upload the mod files here, because this also seems to happen with my seasonal forest biome, downloadable here. I’m just using the default object spawning from the temperate biome, so I’m clueless as to why this is happening.