Zombies and skeletons (possibly other undead) do not burn in the daylight when standing next to a lamppost made from fence posts. The mobs are obviously not underneath the fence, nor are they protected by a helmet, but will still stand there safely until attacked.
I have also noticed undead mobs not burning under other conditions, but this is the only one where I'm sure it can be replicated.
The zombie is clearly standing on that slab. Duplicate of MCPE-32822.