For what it's worth, I ran across a work around when I was playing around with this issue.
While your one character is locked in dialogue, switch to another character and kill one of the 3 new NPCs om tjhe room (I killed Olodan with Astarion). This will force the dialogue to end and combat to start. Once you complete the combat, remaining NPCs behave as if you finished the dialogue leading up to the combat.
Unfortunately, with that workaround, you don't get the benefit of seeing what happens or trying the non-combat alternatives offered in the dialogue. But since I'd already done this recently I'm okay with the result because it means I can push forward with my playthrough.
