I think I've encountered the same bug (or similar, at least).
I believe it is the case that if the boss wanders outside the wildland area, then you're screwed. I've seen it at least once where I killed the boss outside his wildland area, plus I killed all the wildland creatures, and the wildland was forever unusable. I don't know if this is relevant, but the boss was pretty far outside the wildland area, and it seemed in this case likely that he wasn't ever going to go back,.
On a huge map where it took forever to get it done, this was really frustrating!
I have started a new game, and I've seen the boss again wandering outside his wildland area, and I'm wondering if he goes back in and I kill him then, if I will get the quest completion and use of the wildland this time around.