I don't know if this is a bug or a huge mistake. On the experimental settings I found that zombie mob spawners are not spawning in the world gen. Me & 2 friends spent 4 hours with x-ray, night vision & spyglasses in a creative world. We did find 4 spawner rooms with out any spawners, 10 spiders, 4 skeletons, 0 zombies.

Settings: Caves & Cliffs On, Creation Of Custom Biomes On. Seed # -2076823388

Before anyone says try chunkbase, We did does not work with experimental. We also tired with out those settings & works fine.