Experience Orbs occasionally can't pass through upside-down half slabs, but their animations can

I'm not sure what exactly to say, but it happens mostly in a zombie mob-trap I made, where I have two hoppers east of my standing location that feed into a chest, and an upside-down wooden half-slab above the closest hopper, and the thing is that some of the experience orbs don't get through the half-slab, but their animations act like they do, and it CONSTANTLY blinds me until I break the slab or leave... it's annoying as hell and happens EVERY TIME I try and use the trap...

and before any asks, I would take the slab out, but then the baby zombies come in and kill me (happened far too many times) and even when I replaced it with a single fence, they still somehow get through and try and kill me (I went back to the slab as soon as I saw more than 1 got out)