When you have 1 lead in the stack, and you right click an animal (tried horse and pig) the lead will disappear from your inventory, making you unable to bind animal to the fence. You can still right click the animal to get the lead back. If you select another lead item, you can bind animal to the fence. This isn't reproducable in creative, as the lead is not consumed.
I think this happens because right click with empty item stack doesn't check if a lead is bound to an animal, while right click with lead does.