The Burning Lands failed to transform into habitable land in my current game after defeating Delin. My best guess as to why it bugged out is that somehow Delin got pulled far out of the wildland (10-15 tiles outside) and was marauding outside an AI city when I defeated him. Clearing all the lesser remaining monsters in the wildland after defeating Delin also had no effect. I did get the special city fire/growth reward and the fire damage/resist trinket when I defeated him, so that part triggered properly.

Let me know if you need more information, not sure what else I can tell you to help you pin this down.