I lost a shulker in the market just now whilst using a shulker machine. When I pressed the button to extend the piston to break the shulker, the box vanished. My inventory was not full. It did not despawn. I watched it float there, ready to be picked up. When I moved towards it, it vanished... I relogged, thinking it was probably a ghost block and that didn't yield any better results. I checked the dispenser as well and it wasn't in there. And it definitely was not in my inventory... I checked everything extensively before coming to post here.
This happened at -76 80 189 facing west. It's the shulker machine in my shop.
The shulker in question was undyed, unnamed, and empty. Previously, it was full (sans one slot) of gunpowder, which I emptied and placed in a chest as an item for sale in my shop.
I am not overly concerned about this particular shulker box being retrieved and returned to me as it was empty. I just wanted to make the problem known. I would hate to see anyone lose something valuable due to this issue. Maybe it's some kind of bug?
I tried to give thorough details but I'm ready to answer any questions if need be.