It used to be that monsters in chunks outside of ticked areas were not deleted when you set the difficulty to peaceful, so if you visited an area during the night with normal difficulty, left while it was still dark, and then switched to peaceful and went back there, one of two things would happen:

• If it was daylight when you went back, the hostile mobs would still be there and the undead ones would catch fire and burn to death.
• If it was night when you went back, the hostile mobs would still be there and would target you if you were close enough.

Mojang recently made changes that included causing some mobs to despawn more frequently. Perhaps at that time, they arranged for hostile mobs outside the player's ticking area to despawn as well, because the above scenario no longer occurs. Instead, if you return to the area during the day, the hostile mobs are simply gone. Except for phantoms. Any phantoms you left behind in the darkness will still be there when you return, even if the difficulty is set to peaceful.

Steps to reproduce:

1. Select a world you haven't slept in for several days, so that phantoms will spawn in it.
2. In creative mode with difficulty set to normal, find a good sized plain (for visibility's sake).
3. Set the time to night. Watch for a bunch of monsters to spawn, including phantoms.
4. Fly (or teleport) at least 200 blocks away.
5. Set the time to noon. Set the difficulty to peaceful.
6. Fly or teleport back to the plain.

Expected results:
All the monsters are gone.

Actual results:
All the monsters except phantoms are gone. The phantoms are there and start burning as soon as you get within ticking range.