Mobs are spawning on double stone slabs (WORKS AS INTENDED)

A few months ago I had made a battle grid designed for mobs to be able to walk into my battle grid, but not spawn in it, my battle grid was made out of double stone slabs since mobs are not supposed to spawn on double stone slabs.

But recently more mobs then normal started appearing in my battle grid. I recently discovered that mobs were spawning IN the battle grid, which is somewhere they shouldn't be because it is made out of double stone slabs. Correct me if I'm wrong, but on Minecraft Java they should not be spawning on double stone slabs, I'll try to test other double slabs and see if they have the same issue.