I have never seen anything like this before:
Compared to regular AI, zombie AI STRICTLY follows navmesh. So strictly in fact that if it generates mid-air for some reason they will float. And if the navmesh go through objects, zombies will too.
I checked the collisions, they are fine, i see no reason why is there a bump (its hard to see but its there:
Collision is fine:
Edit: I used fences to block stuff, maybe thats the issue…hmmm
EDIT 2: I THINK i found the problem: In short, because navmesh for stairs is inperfect and because zombies strictly follow navmesh, it will never look perfect, zombies may float or go under rendinsts because the navmesh is there.