So I was exploring the Wild Land that has the ice giant in it (I don't remember its name, but I think its title is something like "Guardian of World's End").

I saw how powerful he was and left.

About 30 seasons later, the giant left its frozen home and started attacking my nearby outposts.  Although I didn't expect it, I thought it was pretty cool and had to rally both of my armies to go defeat the beast (fortunately, my Sovereign was a Fire Archmage and I was able to remove over half its hp on the strategic map with a couple spells before engaging the beast in combat)...

I killed the Guardian and decided to wipe out the rest of the wild land so I could claim the treasure:  an altar that granted mastery of water magic.

I killed everything in the wild land, but the area would never register that I had done this (i.e., the greenish, wildland borders remained and i couldn't build an outpost to harvest the magical altar).  

I spent a while thinking about what the bug could be and I came up with a couple possibilities:

1.  The Wild Land simply didn't register that I had defeated all of its denizens (perhaps because the code isn't counting properly, perhaps because I killed the leader outside the Wild Land itself...  It was in my territory after all)

2.  The monsters that lived in the Wild Land wandered off, never to be seen again.  Since the leader did this, I'm assuming that the little minions can as well?

I spent about 50 turns searching for anything that could have come out of there but was unsuccessful.