When I loaded by world, the leads tying my horses to fence posts were gone. Later, when I tried to retie my horses to the fence they leads would not work. I had 13 in a stack.
To trigger this bug:
1.) Load a world with an animal bound to a fence post.
2.) Right Click with more than one lead in your hot bar, on a horse.
3.) Right Click on a fence post with an animal attached to the other side of your lead.