Zombie & Skeleton don't burn on slabs

Zombies & skeletons will not burn from sunlight if they are standing on a slab.

What i expected to happend:
Cleansing fire enveloping the undead spawns and bringing them back to the land of the dead.

What happend:
They stod atop of some halfslabs and did not burn.

Steps:

• make a container with halfslabs.
• fill container with mobs.
• wait for daylight.

NOTES:

• Mobs would still burn if they started jumping.
• Also occurs in survival mode