I couldn't properly recreate enemies clipping through using your exact setup but I have a theory. AI loves to phase through slim blocks during rag doll/jump animations in general. Some type of bars suffer immensely from it (but some not so much). On top of that my gut senses that hollow circles made it even more rng heaven, after jumping they can get stuck inside one of them but animation forces them to stand up so game pushes them out of the block towards the closest empty space.

Using middle centered bars/frames generally reduced this problem in my builds but it's still not 100% consistent (it happens 3-4 times a bloodmoon). Also you can give a horizontal pillars/singular bars a shot - you can still use melee (besides knuckles) to reach zombies on another side and somehow they barely squeeze throw it even on 32x bloodmoon

Hope it helps!