Alright so a friend of mine was flying me to a city and then he decided to take off his Shadowform and ended up losing the mount but keeping the flight + carrying me in his hands. It's pretty funny and both of us saw it including our friends. I can see him floating and so can others so it's not just a visual bug on client side. Anyways I thaught I would share.
Here is how to do it.
While transformed with "Polyformic Acid Potion".
Go into Shadowform.
Mount a passanger flying mount (Sandstone Drake)
Unclick Shadowform.
There ya go, you are now flying without a visual mount and other can see you floating through the air.
Here is a screenshot.
Here is a video
Enjoy