The Silent Cave : Incomplete after Boss Death

I killed the Boss in the Silent Cave encounter where he was empowered to have 4 greater essences.

I killed him on a Storm Weaver Sorceress and the final hit was using the skill lighting warp.

After it's death it still says I need to Defeat the Imprisoned Boss.

Don't know if this is a common bug, or a unique interaction with that skill.

I also had this happen on my Amazon. Still 2 points short of 40 because of it. Womp womp