I've been experiencing this issue for many times. (Named)Mobs or mobs in general (like farm cows) despawn when I go offline playing and go back to online mode again. Even named villagers keep despawning despite having houses. The same goes for my horses. Once I go back to my horse, I can only see a leash on the fence but with no mob latched on. I tried restarting both the game and my phone but nothing happened, they were still gone. It seems like the mob data aren't saved in the storage.