Because I found a game-breaking bug in Trespasser where if you approach the magic in the ruin with a party member it gives the new anchor ability to that character rather than the Inquisitor, and of course the party characters can't activate it.
So I thought it might be a similar case, where it gives the ability to a party character that doesn't have the coding for it. Meaning it just gets lost in the program.
Maybe it's the party configuration. Maybe the banter is messing with it. Maybe it's the response you choose. Try reloading to before you talk to the spirit, and then go back with Cassandra, Sera, and Cole. That was my playthrough, and I think I asked Cole's opinion. Then I said the spirit could rest now. And then I got the new ability. I had fought the Despair Demons before this also. Doubt that has anything to do with it though.
Also, try going back to the spirit and speaking to it alone, without any party members. That will definitely prove one way or the other if the ability is going to a party member or not.