Villagers Despawning and not breeding with food

When I was playing on a realm with my friends, each of us experienced our librarian villagers despawning after leaving our base. After that, I decided to create a survival world in the most recent snapshot. I gathered some villagers, and started to breed them. They soon disappeared after I went into the end. The only ones that were left were 2 in the breeder, I gave them food, and no villagers for 2 hours. Am I supposed to name tag them? I thought passive mobs didn't despawn??? Thanks