After the most recent update to Let's Play Europe I started a game as the Ottomans, and noted that with the "Import" setting for Resources there were two salt resources generated on flood plains, making them impossible to improve as mines can't be on flood plains (I guess).

I sorted it out with the Map Cheat Mod, and that tool does confirm that floodplains of any kind is an invalid terrain type for the resource. So I guess that it's either a generation bug that generated floodplains under a "fixed" resource, or just a mishap. Not sure if the terrain generation is fixed for the map as well or if floodplains vary based on game seed.