- Picked up a key to open a door (one of those stone things). On my way to the door, I found the horadric portal. Entered it, cleared it, and came back out. The key drops when you change zones. Upon coming back, the key is by the portal but you are unable to pick it up due to the interaction with the portal. Since the portal doesn’t disappear, the key is no longer able to be picked up because the portal is blocking it.
TL;DR - you drop dungeon door key when entering the horadric portal. When coming back from it you can’t pick it back up because the horadric portal is blocking it (portal doesn’t disappear).