Passive mobs despawn when you leave the chunk.

I'm not sure if this is a problem with my client or the server, (It happened on a server) but when I leave the chunk after setting up animal farms the animals are all gone. I retested this in several different locations and on several different servers, and every time I leave the chunk there seems to be a chance that the mobs despawn when I return. This happens with chickens, cows, mooshrooms, pigs, tamed horses, and untamed horses. This makes it insanely hard to do anythg with animals.