So I've posted about this not unlocking myself, and seen quite a few others with the same issue. However I've found the solution. It seems that when HoW dropped, it stopped counting existing exotic weapons as exotic. This hadn't unlocked for me despite fully upgrading my Mida, Thorn, Thunderlord and P&T. But when I fully upgraded my Queenbreaker's Bow, the achievement popped instantly. Nothing I need help with, but I'm posting it here for those still having this issue to see. In summary, upgrade one of the Elder Cypher exotics to pop the achievement.