Villagers keep disappearing even if we didn't leave.

I have been playing since May of 2021. I like to play multiple players on a single world. When we gather each type of animal and large numbers of villagers in our main base then save and quit, when we log back in many of the mobs have despawned. It happens most often to the villagers even after they have been named, bred, and traded with. It also happens to the animals. We will have a sheep of every color completely lit up and completely enclosed in fence, and when we log in half of them are missing. It's hard enough to find villagers without having them stolen from us. We are considering finding a new game to play together because we are frustrated with having to start over all the time.