Passive Mobs Despawning When Not Supposed To

So I’ve been building a kingdom in Minecraft pocket edition and my animals have been mysteriously disappearing as of late. I have separate pens for my cows, pigs, chickens, and sheep and even have stables for my horses and llamas, all of which are completely surrounded be fences(stone brick) and are all less than 20x20. As of late, it is quite frequent that a few disappear at a time.