I had an issue with Zone of Control when capturing an enemy Outpost that I believe was on the edge of an enemy city's Zone of Control. I captured the Outpost, then the next turn the Outpost was silently reverted to enemy control, even though my unit was on top of it. This meant I was now in enemy territory and could be targeted by enemy spells. I reloaded, and was able to move off the Outpost and back on, which gave me control of the Outpost and also give me Zone of Control for the Outpost and for its Zone away from the enemy city. (Towards the enemy city remained enemy Zone of Control).
Not sure if the enemy city Zone of Control expanded the turn it flipped the Outpost I'd captured? If it's intended that city Zone of Control can flip Outposts then it certainly shouldn't happen when you have troops stationed on the Outpost.