The conversation drops after initial statement by Ambassador Hylgard.

Changing to any of the three different Sapadal decision has no affect on the bug. Both regular leaders of Thirdborn and Fior are there.

I believe the bug comes from Kostya. I Saved Solace Keep, but I did so in "stealth" and thus never fought/killed him. During the Siege of Paradis quest Warden Kostya appears instead of Warden Mihala.

Hylgard actually refers to Mihala with Kostya standing right next to him.

It seems Mihala is supposed to being speaking after Hylgard finishes, but because she's not present the game just drops out of the conversation. I assume that the game makes Kostya the leader of Solace Keep if he is alive as he becomes hostile if you choose to save Solace.

I have the same issue. Already reported the issue to Obsidian. I currently have no patience to go back to hours of content just to kill Kostya. 

Here's my bug report....

Steps to replicate:
1. During the "Our Dreams Divide Us Still" > "Enter the Garden", use "Shadowing Beyond" skill while fighting with Kostya and open the Sluice gates.
2. Do not kill Kostya and simply leave to report back to Mihala.
3. Proceed with the main quests until "The Siege of Paradis" > "Speak with the leaders of the Living Lands"

Expected: Mihala is the representative of Solace. She is also the name mentioned by Hylgard. The dialogue with Hylgard cuts off suddenly.
Actual: Kostya is the npc appearing in the quest area

Issue: Due to the sudden cut in dialogue, i am blocked from making the choice needed to complete "Speak with the leaders of the Living Lands"

THIS STILL APPEARS TO BE BUGGED AS OF 1.24

Now when you walk back into the frame Hylgard begins his lines once again, but it also drops when he turns to what is supposed to be Mihala. Restarting the conversation a 3rd time gives the same results, and so on.

How is this still not fixed?!?! Played Monster Hunter Wilds came back to try finish my playthrough and it's still bugged after two weeks. Pretty incompetent to not have a game breaking bug like this fixed after two weeks. 

It's been fixed. I just checked.