Ok, this has been bothering me forever, so I did some testing. Here's what I found:
1. Wildland monsters ignore wandering radius, they have their own movement logic. So it doesn't matter whether they spawn from huts or get placed directly on the map.
2. When an army enters a wildland, the guardian will move towards it. When the army leaves, it will move back towards its starting point.
3. Each AI turn the guardian will calculate the shortest path to either the invading army or the start point. If the shortest route leads outside the wildland then the guardian will take it.
4. Once the guardian leaves the wildland it might start moving around the entire map. And the quest won't complete if a guardian is killed outside their wildland.
It's possible to have happen with any guardian except Abeix (who can't move). But it should be possible to eliminate this behavior just by changing the wildland stamps, e.g. by placing and removing a few forests then the pathfinding AI should always keep the guardian inside.
EDIT: Looks like it works, with a couple of stamp changes and MovementCostOverride modifiers it's possible to prevent guardians leaving their wildlands in 99.9% of cases, without affecting any other behavior. The fly in the ointment is that the pathfinding won't consider a move into a space containing another unit, so if it's completely surrounded by other units it can still leave the wildland. But this is a very rare case. I'll put a mod up if there's still any interest in a fix for this.