If you are leashing a mob with a lead, the inventory slot which contained the lead will be empty. If you then select a fence item in your inventory while still leashing the mob and leftclick on another fence which is already placed to tie the mob to the fence, your fence item of your inventory will be gone (or the amount will decrease if carying multiple fence items in the slot).

The mob is successfully tied to the fence you've left clicked on, but your fence, you've carried is also gone. It is not visible! But you will see some flickering lead on the fence block where it is tied on. You can get your fence item back if you destroy the fence block where the tie is on. After the block gets destroyed, you will get 2 fence items dropped.