A variety of different mobs have been despawning for no reason. Farm animals in small fenced in pens decrease in number over time. Tamed horses despawn including when they are on leads and have been named. Tamed parrots have also despawned whilst being on leads and named (the lead remains attached to fences but the animal vanishes). Foxes and skeleton horses have had the same issues. Wolves do not seem to be having the same issues as other mobs. It has been making the game very frustrating to play. Bees have also all mysteriously vanished. 

I have included a picture of two fences each that had a skeleton horse attached (tweedle Dee and tweedle dum, they were both named and both attached to separate fences on separate leads) as you can see in the photo- only one named skeleton horse remains but both leads are still there.