Hostile mobs are no longer spawning when switching from peaceful back to normal/hard mode

What I expected to happen was...:
Hostile mobs would spawn

What actually happened was...:
No hostile mobs spawned. Peaceful mobs still spawned and were able to be killed.

Steps to Reproduce:
1. Place realm in peaceful mode for whatever reason (i.e. younger sibling wanted to look at your world)
2. place realm back into normal mode via configure options