I was AFK for about an hour at my automatic villager breeder. The villagers are protected by fences. When I came back from AFK, There were no villagers except two zombie villagers holding carrots. When I had converted the zombie villagers to start the breeding from the beginning, I entered a r]test world. I spawned a lot of villagers in the fenced off pen and spawned zombies on the outside. They were able to attack the villagers. They can also attack the villagers through stacked up fences.

To reproduce, go into a creative world and spawn a lot of villagers in a pen of one high fences. Then spawn lots of zombies on the outside. The zombies will attack the villagers. After that follow those steps again except this time using two high fences.