So the only remaining possible gates are either Raids Launching or Artifact Knowledge/Power. My thoughts were always that it's still the charm, and the charm is still a random drop. Just now it can't drop unless you meet the requirement. Seeing that Ret Ashbringer is gated behind AK leads me to believe that ours is the same.
So each AK level I'm just going to go farm a few more, and if that doesn't work I'm sure someone else spending way more time will figure it out.
Edit: Theory works for AK3, if EU has it unlocked now, see Nay above- can test when NA servers are up.