Mobs cannot seem to pathfind from grass paths properly. I first noticed that after I replaced the gravel in a village with paths, the villagers wouldn't go up the stairs into their houses and got killed at night. I then did a proper test (see attachments) that showed that this affects zombie pathfinding too, and likely all pathfinding as well.

In the first video, you can see that the zombie sees the villager, walks all the way up to the stairs, then stops. Even though I can walk up the stairs, and the zombie can if pushed, he will not do it on his own. I then spawned a bunch more zombies, and while they all pushed each other up, the last zombie got left behind and wouldn't climb the stair.

The second video also demonstrates this with full blocks. The zombies won't jump to get to the villager. However, if I replace a block on the path layer with a full block, they will go to that and then jump.