When exploring areas in daylight or around dawn zombies and skeletons will appear but not burn. They are not wearing armor of any kind, just your base mob. Seems to happen most when exploring in one direction after a good distance and the area generates fresh mobs. So far only seen in the Plains Biome.

After staying in the area for a day cycle the next round of skeletons/zombies that spawn burn in daylight as normal. But when the area is first approached they seem immune to it. I couldn't find the bug listed elsewhere so decided to create a new entry.