After a legendary chest has been looted, it respawns twice.
The first respawn happens after 15 minutes, and this chest cannot be looted or interacted with in any way.
The second respawn happens after 30 minutes, and this chest is a normal legendary chest that can be looted and destroyed. However, it does not replace the first chest, they both occupy nearly the same space.
The real and fake chests are offset from each other very slightly, like probably in the sub-pixel range so it isn’t even possible to see. But, because they are offset, you can interact with the real chest from certain angles. Move around the chest and try from every side until you manage to get the real chest.
Once looted, the real chest will vanish like normal, but the fake chest will stay there permanently until server restart.