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Actually, I sometimes experience something similar, when I've just started creating a map, and through some kind of "magic," it eventually appears in the list. And it's possible that the absence of a town for one of the players could affect this. Accordingly, either add a town or change the victory conditions (remove death from prolonged homelessness or make other changes). I don't fully understand how this works, but I repeat, this "magic" has helped me personally.