Mobs no longer spawning on pressure plates.

Mobs (Endermen) are no longer spawning on pressure plates.

What I expected to happen was...:
We spent 2 hours making an Ender Ender Enderman Farm that should have worked, and it has.. in the past.

What actually happened was...:
Nothing happened, no entities spawning and there was no other space for them to spawn in in the world

Steps to Reproduce:
1. Get into an empty world or empty part of the end
2. Place a block with a pressure plate on it
3. And no mobs spawns on it, EVER!