Animals disappearing/despawning

Hi, 

A now common occurrence on my minecraft world is that my tamed animals and animals in pens (farm like situation) seem to disappear/despawn after a few days. There is no way they could have just escaped from the pens, all fencing and roofs intact. This has happened to me numerous times, probably about 5 or 6 different occasions on this world of mine. 

Please fix this glitch ASAP, as it makes the ability of taming animals and having a farm quite useless.