Villagers disappear when you downgrade your minecraft version

In 1.11 prerelease I have a large number of villagers. When I changed to 1.10.2 or lower all of the villagers are gone.

Steps to Reproduce:
1. Have villagers in a version above 1.10.2
2. Downgrade to 1.10.2 or lower
3. All the villagers are gone