If you discover a new resource in an already improved tile, the resource is visible if you continue playing. But, if you save the game and then load it, the resource becames invisible. In the images you can see the coal in a farmed tile when it is discovered, and coal invisible after loading the savegame.
I think the reason may be the order of objects drawing when loading a savegame. Perhaps the graphic for the coal is loaded before the graphic of the farm, and thus the coal is invisible after loading the game.
I have attached the savegame before the discovery of scientific theory. I'll post the savegame after scientific theory in a response to this thread (it seems you can only post a maximum of 3 attachements).
Edit: Removed the savegame due to quota issues
I think the reason may be the order of objects drawing when loading a savegame. Perhaps the graphic for the coal is loaded before the graphic of the farm, and thus the coal is invisible after loading the game.
I have attached the savegame before the discovery of scientific theory. I'll post the savegame after scientific theory in a response to this thread (it seems you can only post a maximum of 3 attachements).
Edit: Removed the savegame due to quota issues