Monsters Still Appear After Changing To Peaceful, But They Can't Do Anything

What I expected to happen was monsters to disappear after changing to "Peaceful", but what actually happened was that the monster stayed, but it couldn't do anything.

Never happened on other versions.

Steps to reproduce:
1. Make a custom generated world
2. Find a monster
3. Turn it to peaceful
4. It sometimes glitches and the monsters stay, but can't do anything.