1. Make void world
2. Wait for/Make it night or create a mob spawner
3. No mobs spawn

This was apparently intended behavior, although I don't know why I didn't just make a config option. This will be fixed in 1.12.2-2.3.0.4.

Thanks. I'm constructing a modpack and planning to use the void world type for it, so it needs to be able to spawn mobs.