I don't think I run any weird (or many) other mods (latest unique 3-4 comp, this scotland mod v6, improved city view and top panel replacement). I guess it could be the improved city-view but I thought that mainly compacted the buildings and great people parts on the right side of the screen.
It all looks normal in the various world view. But if you enter a city (included as a thumbnail) it looks a bit weird. In Glesca here you have a Dun to the west of the city on some stone in a desert and it has the deep blue graphics as not belonging to this city.
But it doesn't belong to any other city either. Non is close enough to claim it or work it. It has the same deep blue border as tiles worked by other cities (like the fish up in the north eastern corner that is worked by another city (circle-arrow) or the mine across the lake to the north west that is worked by another city -- sort of obscured by the city name here).
So who does the tile, the desert stone Dun in to the west, belong to if it is not Glesca? The closest other city center besides Glesca is four or five tiles away.
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As noted it's only a problem if you build the Dun out in the unclaimed lands. It's not a problem if you build one what is already inside your borders.
It's not really much of an issue, you just have to manually go in and select to work the tile. But if you forget it will remain un-worked and never be selected when a new pop is born etc.