After tethering an animal to yourself, you must have a rope selected to tie it up to a fence.
This means that to tie up an animal, you'll need at least 2 ropes, even though the second one isn't consumed.
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Duplicate of MC-13754.
Not a bug i would say, its logical for you to have the leash in your hand while you tie it up, and also only 1 leash is consumed.