After beating an AI nation in a war, they were eventually classed as eliminated. However, this triggered while they were sieging an independent city. As can be seen in the screenshot, their units are still classed as being in the siege, and I cannot interact with them as I do not have enough war support to declare war on them, as they are eliminated and cannot be interacted with. This means that while I could build an outpost in the territory, I cannot build any districts, or move freely, through it, as it counts as being a battlefield, and I cannot get past the control zones of the AI units. At the time of the screenshot this has been an issue for over 150 turns. I am not certain what exactly triggered the faction eliminated trigger, as I was not the one who did it, I can speculate that another AI faction took their last outpost, while they were sieging the independent city, and therefore counted as eliminated.

Please let me know if you need any further information.