Zombies/Skeletons do not burn in daylight when standing near (not under) fence posts

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.