What I expected to happen was...:
I expected that vanilla village design be so that villagers can't get trapped in areas.

What actually happened was...:
A fenced in area is placed right next to a farm. Villagers jump up on the oak logs of the farm and then walk over the fence next to it falling into the fenced-in area trapping themselves. The fence should have 1 space of air next to it on both sides to prevent villagers or any other mob from crossing it. Or the fence should have the same properties as rails that prevents mobs from walking on them.

Steps to Reproduce:

1. Create a world with the seed 6407707231971289126
2. Teleport to the coordinates x: -224, z: -348
3. Watch villagers jump up on oak logs and then fall into fenced-in areas and get stuck if gate is closed which by default, it is of course.