Zombies have trouble with finding villagers, even if there is a clear path to the villager they still are unable to traverse the entire maze to get to them and simply get stuck on a wall like they are now. In the image are invisible zombies wearing some wool on their heads, and on the other side of the wall is the villager. There is still a path to getting to the villager behind them, but instead as seen in the image, they are stuck on the wall and never move from the spot other than wandering