Passive mobs and name tagged mobs despawn sometime between logging off and logging back on. Over the last two months 13 named mobs have despawned (Fish, Horses, Turtle, Panda, Sheep, Chicken, Zombie, Slime) as well as multiple nameless mobs (Cows/Bees) the bug doesn't seem to effect Dogs or Cats. All mobs that despawned  are within 200 blocks of each other and other players mobs don't seem to be despawning. (Note the mobs are not being killed they are despawning)
Not positive on how to replicate the issue but it seems to me having multiple mobs in a small area may be the issue. Mobs don't despawn while playing only between being playing sessions.