Iron golems spawning on bottom slabs and stairs

I have built an iron golem farm, and iron golems are repeatedly spawning on stairs and bottom slabs. They do not fall down from other blocks onto the top of them. they are not spawned below the stair or slab on the block below. They are 100% above the stair and slab when they appear. This is in 1.15.2 but your bug tracker won't let me select the version