steps to replicate: 

1. attach animal to lead (animal type does not matter)
2. place fence post on ground
3. select fence post in hotbar (I believe it must be one, not stacked fence posts)
4. right click on the fence post to attach the animal that is on the lead

Results:

The fence post in the hotbar will become invisible, but will reappear when placed on the ground

• visual glitch only-