I know that from 14w30c atleast bred villagers will despawn. You can't reproduce it quickly. It is on SMP server. The bred villagers only despawn and the places they stand in are zombie proof. After a while bred villagers were despawned, the villagers that were not bred, but restored from infected are alive and not despawned. You can find this for sure in the code if you can look into this. I can not do that, but if someone can they can prove it too.