Iron golems able to kill zombies through one layer of blocks down.

We have built this awesome "Zombie treatment center" in our town in which we resurrect zombie villagers. This room happens to be one block under the ground in our town, and we also have iron golems defending the town. Sometimes, they just kill the zombies we're trying to resurrect through the one-block layer they're standing on. So far, I know they can do this through Quartz blocks, wood planks and dirt.