I have the same problem. Similar to Suffolk_Memeplex, my main character is a lolth-sworn drow warlock, and I previously helped the goblin Sazza escape. I told Minthara I'd annihilate the grove, came back, and did just that. I've killed all druids, opened up the spiral staircase down to the secret area, cleared the long tunnel to the back door, the woods around the main gate (Tieflings), the Tiefling singer over near the small beach, etc. Really not any living thing around except green-outlined goblins.
One thing that I wonder is this. I only blew the horn and opened the gate for the goblins AFTER I had killed all Druids and Tieflings (on btw I killed the adventurers too, including Liam over on the rack in the Goblin camp). Maybe since I haven't killed anyone since blowing the horn and opening the gate Minthara isn't like "reading" the situation? She's stuck in "hunt them down" response only. And again like Suffolk_Memeplex my journal says:
 Return to Minthara. Tieflings and Druids alike lay dead. The grove belongs to the Absolute. Minthara will be pleased.
My party the whole way has been me (lolth-sworn drow warlock), Shadowheart, Astarion, and Lae'zel. We're Level 4 now. One of us has the brand. Oh we DID kill the goblin priestess but we did so in private
without making enemies over at the goblin camp. (Had to destroy that pesky war drum).
Suffolk_Memeplex did you figure out how to get this to work?