I was exploring a mountain on a realm when I saw 3 zombies under a group of trees and I needed XP so I walked up to them but then backed away when I saw a creeper. .as I backed up the zombies followed out from under the trees but when it completed daylight neither of them caught fire. They were not wearing hats or any kind of armour.

This only happened on realms. I tested this on a singleplayer world and the zombies burned.