I'm having the same problem. No dialogue will trigger in Act 3 when I first see Thulla in the cave near Rivington.
I have no mods, great PC, and the game has not crashed even once. This is the first bug that's causing me to wonder if some quest may break later on....
Help, Team Larian! Does it matter? Is there a workaround? Will it cause issues later on in the gnome-related quests of Act 3?