Slimes appear for 1/2 a second in Peacefull

In my creative flat world, on peaceful, I was flying and looking around until on the side of my screen, I saw a slime for a second. But it disappeared right away. I think this is a bug.

What I expected to happen was for there to be all hostile mobs to be turned off in peaceful.

Steps to Reproduce:
1. Fly, in creative, in a flat land world.
2. Set your difficulty on Easy.
3. Fly around again, without hurting mobs.
4. Set your difficulty on peaceful.
5. Fly around, and you will see, for a second, slimes.