This bug or lack of implementation may be inherited from unmodded BTS. Basically the AI doesn't seem to consider moving workers to its culturally owned tiles on landmasses where it has no cities. When I gift a worker to the AI or create a worker in WorldBuilder on this isolated tile, he builds a road and an improvement for the city; or if the tile has a resource on it and is not workable by any AI city - a road and a fort. After that, if the AI has an unoccupied transport ship, the worker sails elsewhere.
So the implementation or fix needed for most cases is simply making the AI consider shipping workers to such tiles.
There is another much less common issue here: if a resource on a tiny island is workable by a city, the AI builds an improvement on it and does not seem to consider building a fort instead. It also doesn't build forts on other tiles of this island to connect the resource. The AI does build roads on its unworkable island tiles, but these roads do not connect the resource without a fort. I suppose making the AI understand the fort mechanics could be more difficult to implement. Fortunately, this is a not too common edge case.
So the implementation or fix needed for most cases is simply making the AI consider shipping workers to such tiles.
There is another much less common issue here: if a resource on a tiny island is workable by a city, the AI builds an improvement on it and does not seem to consider building a fort instead. It also doesn't build forts on other tiles of this island to connect the resource. The AI does build roads on its unworkable island tiles, but these roads do not connect the resource without a fort. I suppose making the AI understand the fort mechanics could be more difficult to implement. Fortunately, this is a not too common edge case.
