This happened to me today while my inventory was full. I made room for the Radiant Vambrace Skin and clicked to get it out of overflow into my inventory. No Large Achievement Chest to be seen anywhere, not even stuck in overflow. I tried to make some more room in my inventory but nothing came up. Tried to relog but that didnt do anything either. It is lost to me and it’s a shame since the rewards in that chest is pretty nice.