After building a village about 30 blocks below the zombie trap, the zombie trap stopped functioning properly and when I looked inside in spectator mode, zombies were gathering towards the villagers. No matter how many times we turned off the zombies, this situation did not change.

We then experimented with this.

1. Place the zombies in a space about 5 blocks wide.
2. Create a room next to it and generate villagers in it.

Prediction: Zombies will not detect the villagers.
Result: Zombies can detect the villagers.