This is a visual bug; it doesn't affect game play.
I share an island with the winner of the Hydra game. In the island view, his towns have the dome picture.
If I go to my town on the island I share with him, then open the saw mill, then go to one of my other islands (it has to be one of my islands, so the saw mill window stays open), the domes show on my other island, even though they are not towns of the winner of the Hydra game. One of my islands is mostly empty, and I can even get the dome to appear on an spot that has no town.
