Mushroom fields can fail to generate.

What I expected to happen was...:
I would find a mushroom field filled with mycelium and mushrooms.

What actually happened was...:
The island was underwater and no mushroom island features spawned.

Steps to Reproduce:

1. Generate a world with the seed 3632783455964709019

2. Teleport to -5748 63 -1531

3. Press F3 to see you are in a mushroom field with no mushroom field.