When you defeat the Echo Boss and go to the next floor, after opening the big chest and looting the legendary weapon, you need to exit to the main menu, without exiting the chest floor, then you just need to click on continue form the main menu and thats it, you just have now a new fresh chest to loot
1 Like
I happened upon this bug today as well, but it seems like it actually also happens if you take the elevator to the top floor after looting everything but not moving anywhere and logging out. When you log back in you will have been saved down below with all chests respawned and no longer looted. New legendaries and other loot have re-appeared.