The Ender Dragon disappears when you start the world in Pacific difficulty and If I changing it to easy or normal, it still does not appear

What I expected to happen was...:
I expected that the Dragon was there only entering in the End

What actually happened was...:
The Dragon Doesn´t appear 

Steps to Reproduce:
1. Create a Normal World, in Normal Difficulty

2. In world, change to Peaceful Difficulty

3. Go to the End by a EndPortal

4. There´s no Dragon

5. Change to Easy or another Difficulty

6. Still not appearing