Zombies stop attacking villagers that are against fences, panes, or bars

I was testing out if a single zombie could kill a whole bunch of villagers when I noticed something strange. The zombie would sometimes stop attacking villagers that were pressed up against the fence.
Later, I did some more testing and found out that sometimes if the villager moves or is bumped, the zombie would start attacking again. Strangely, if you hit the villager then bump the zombie, the zombie will attack again. Sometimes the zombie would start attacking again on its own, but that was rare.
I tested it with fences, glass panes, iron bars, and glass blocks. It happened only once, on the last villager, with the glass blocks.

Steps to reproduce:
1. Play in creative mode.
2. Construct an enclosure that has a border of fences, glass panes, or iron bars and will keep the zombie from burning (such as a soul sand floor).
3. Use spawn eggs to spawn some villagers and a zombie.
4. Watch the zombie attack a villager, then stop if the villager is up against the edge of the enclosure.