Baby zombie pigman fitting through 1/2 hole (slab)

Recently made an overworld zombie pigman farm where they are placed in a basic xp grinder box setup (but 2x3 trap area as opposed to 2x2. Baby zombie pigman can fit through half slab area. Vanilla, survival, multiplayer.

What I expected to happen was...:
baby zombie pigman to stay behind half slab of cobblestone

What actually happened was...:
Once aggro'd, the babies would escape through the half slab that was within the top portion of the block (aka if you take two half slabs and make them one block, it's the half slab on top) (Could even see the AI walk away from me to go to topside half slab). However, this may also be caused by the setup: bottom slab on left, top slab in middle, bottom slab on right (on the area with a length of 3 blocks). The entire trap spot is secured until the baby zombie pigman become angered.

Steps to Reproduce:
1. 2x3 block area for zombie pigman, hoppers on bottom
2. each corner is a solid block
3. cobblestone half slabs fill in (see above: bottom, top, bottom setup on side with 3, on side with length of 2: the slabs are on the bottom)
For easier understanding, see attachment: pic included NOTE: still occurred while nether fence was once other solid block (lime wool/netherrack) (bit of a pain to visualize). Also, apologies on the overly descriptive summary, I know sometimes that programming is a mesh of "there is no reason for this obscure thing to interact with this other obscure thing".