Sometimes the chest has been opened within the last 30 minutes but the chest will still be visible when it should have disappeared. Also try crouching and spamming the interact button on each side, sometimes that works.
This is not a solution, because the chest only shows again after 30 minutes. The problem is that it’s not lootable again after 30 minutes. This is the same for all legendary chests. You can camp them after use and they will bug every time.