Villagers despawning - including my named sheep

For the whole game: Most of my worlds have villagers in them. When I join in one of my worlds, I noticed that some of them are despawning. This doesn’t occur all the time. I kept putting them back but they did it again. Same for survivals, but it only happened once and I can’t put them back because spawn eggs are uncraftable. 
For Villager Races: I have created a world for villagers to race. Every day I keep going into my world some of them are despawning. My sheep named Announcer, which is used to inform the villagers about something, despawned twice. I kept putting them back but they did it again. How do I stop this?