Happens after you lead them to the pen & go off to find another cow/sheep/chicken. When you come back, there's nothing there! You can hear the sheep/cows but you can't see them in the pens - therefore can't breed/kill them. I have tried stone walls and fences & same issue, although doesn't happen to ALL pens you make, seems to be in certain areas of the map. Very annoying glitch, especially if you're on survival mode & it's your first/only livestock pen. I have to find another area to place my farm, any help would be appreciated! Thanks.