When you scaffold to the top of the world and right click on the top of it to bridge outwards with scaffold it says that you can't build higher in the action bar and looks like it places the scaffold to the side like normal (it takes a scaffold from ur inventory, places it in the world) and you can stand on the scaffold and stuff, but whenever you right click on it it disappears. When you relog all of it is gone and u get ur scaffold back, so I guess its some kind of ghost block.