I just played a couple games on the Wilderness DLC map and in both cases encountered a bug where I completed the quest requirement to clear a Wildlands and it didn't clear.  The first game, I had the problem with Vetrar, the Ice Guardian.  He came wandering out of his wildland and I killed him as he was approaching one of my cities.  The quest completion trigger is his death, and he was definitely dead, but the wildland remained wild.  I was able to clear another wildland in the same game and it triggered fine.  Next game, I had a wildland which was a "kill all monsters in the wildland" to complete and I definitely killed everything in there, from the dragon and Shrill Lords down to dozens of shrills.  I sent multiple units in to make sure that I could see the whole area at one time and confirm, there was nothing alive in there except my units.  I scoured the surrounding area for any stray units that might have wandered off but there was nothing and the quest never completed.