Armoured zombies don't burn in daylight

Have only noticed this in Creative mode so far. Created a bunch of zombies using a spawn egg and one of them was armoured. I then set the time of day to day time and they all burnt, except the armoured one (see pic). There was no cover, I used a flatland map and he's exposed right outside in the middle of a field.
Unsure if this affects Survival, etc.