Cows (and presumably other friendly animals) despawn randomly

I was AFK for about an hour at an automatic farm 35 blocks away from a 5x4 pen containing about 40 cows. When I came back from AFK, Iwalked past my cow farm and noticed that there were no cows. I re-logged and there were still no cows. I explored the surrounding chunks and they had not escaped

I am not sure if this can be reproduced or was just a one-off