Mobs spawn within double half slab (full) blocks

I'm having an iron golem farm.

Some of the walls were made of double half cobblestone slabs (bottom and top one), to form a full block.

As a result, golems consider that block as not full block and allow spawning within it, when the bottom one is a normal, full (spawnable) block.

The new golem just continue wandering within the row of double half slabbed wall, like it's not a full block. (Breaking the farm, due reached golem limits)

I haven't checked if the golems would be able to breathe within them too, as my walls are only 2-high.