mobs are not walking over bottom slabs with no block under them.

What I expected to happen was...:
mobs to walk over bottom slabs/see them as solid ground.

What actually happened was...:
mobs don't see bottom slabs at all, will try to jump up walls with a bottom slab on top or will see a gap where a bottom slab is.

Steps to Reproduce:
make a bridge made out of bottom slabs and get a mob to path over it, (zombie following you in survival mode, villager running from zombie, villager pathing to hungry villager) and the mob wont walk over the bottom slabs.