Some mobs will not despawn after switching from a non-peaceful mode to peaceful mode at lower render distances.

How to recreate this bug:

1. Load up a superflat world(you can do this with any world, but superflat worlds make it easier to spot the bug).
2. Go into creative to make it easier to spot the bug
3. Switch the render distance down to 2 chunks
4. Go to hard mode and set the time to night
5. Once hostile mobs spawn, go back to peaceful.

Most of the mobs will despawn, but some will not and just stay there forever without moving until you get closer to them. They won't attack you, but you can hear their sounds.