Zombie Pathfinding

From the image attached it shows a group of zombies going after some villagers. But the zombies are not taking the path to get to them. As well when I removed the blocks in front of them the just walk of the edge.
And in the next image there is a path (which isn't straight) between zombies and 1 villager, which the zombies are going after but they just walk towards them without noticing the void beneath them.
This is obviously a bug so please could you fix this.

P.S. Only some of the zombies follow the path to get to villager. I do not know why.