My animals have been constantly despawning since I enrolled in the beta. 
After my sheep of every color disappeared and my cows and sea turtles disappeared, I caught new animals. I fed and bred them. Then, my pigs disappeared, and half of my horses were gone (regular and skeleton), as well as all of my donkeys!

I took a break from the game in frustration, only to come back to a single sheep, no tropical fish (but my salmon are intact), no sea turtles, no mooshrooms, no cows, only 1 horse (regular), no skeleton horses, no donkeys, and a third of the pandas I previously kept. The most frustrating part is that the foxes that I spent a couple of hours finding and bringing back with me through the nether, have all-but-one disappeared.

It seems that when I walk away from my animals, they disappear. When I walk to their enclosures to investigate, they are empty!! Screenshots are attached.