Yesterday I played with my friend, and we both came close to the portal transition to another zone. My friend came close and started transition to another zone (as you know it takes 5-10 seconds) to get to loading screen for another zone transition.
At the same time there was a chest close to the portal - I have opened it and a legendary dropped for my friend. He saw that and came close to legendary item (BIS for my build tho LOL, he saw description - Dreamthorn), however he lacked 1 second to pick it up as he dissapeared due to loading screen. I stayed at the same zone near chest, but when he transitioned back to this zone all items were gone.
I think the bug is all items that are being dropped while you are “Entering … zone” state would dissapear if you go back.
Cheers,
Anton