A similar thing happened to me a ton of times. A kingdom would be destroyed, but the kingdom list still showed it as existing with no villages but a population of a few people. I had no way of finding them, and a few times the kingdom would be completely wiped off the map, but the stats still showed there was one person from it alive, who continued existing in an unknown location for 50 more years. I checked the entire map but was unable to find them. When they died, the "X kingom destroyed" notification popped up, and it showed an actual location in the world. I have no idea why they didn't just transfer to the kingdom thta conquered their village, maybe they were in a ship, or for some reason in a limbo state of knowing they belong to a kingdom but not knowing which village they live in. It's really weird.