Sounds like a bug,
my guess is that the Monster AI tries to go after the pioneer that founded the city/outpost (and fails because it no longer exists), resulting in inaction (guardian) or targetting the next closest thing (wandering mob).
Yet the monsters won't attack an AI-run city even if they're within the city limits. The problem seems to be that to the monsters, the AI-run content--cities, developed resources, heroes, other units--are all invisible. It's as though the code for this has been commented out.
In any case, I suspect we'll get it back, soon. Right now, the AI players have a great boost to development. I've even seen individual heroes running around in AI territory without stacks, going right by Forest Drake armies and the like.