Entities Despawning

I play minecraft pocket edition and play in my friends world a lot. But recently, when I join all the entities in the game keep despawning, including our mending villagers that we worked very hard to find. A few days ago, we went to search for more villagers. We managed to find two but the next morning when my friend checked, the villagers disappeared. This problem has never occured before and i hope you can fix it. The world we play on is not realms or server