Ghasts spawn in peaceful

How to reproduce

1. Spawn a ghast with a spawn egg
2. Switch to peaceful mode
→ The ghast is still there

Original description

I have been playing Minecraft yesterday and I have decided to build something inside the nether. I have entered the portal while on Normal difficulty, and i already knew that constantly hearing ghasts will drive me crazy, so i have switched difficulty to peaceful but the ghast did not despawn. All other mobs have disapeared but not them. By using commands i have killed all ghast inside the loaded chunks and they couldn't spawn anymore but entering further chunks, the ghast were still apearing