Hostile mobs not spawning on snapshot 19w13b (not on Peaceful)

Put the summary of the bug you're having here

What I expected to happen was...:
Hostile mobs should spawn on difficulties easy, normal and hard.

What actually happened was...:
Hostile mobs not spawning naturally (they spawn if I spawn them).

Steps to Reproduce:
1. Put a step by step guide on how to trigger the bug here
2. ...
3. ...