I've come across a bug and I'm not sure if anyone else has reported this. I'm playing on a Giant continents map. So far I have come a across two locations on the same map where there are three adjoining tiles of impassable Grassland terrain. An image is attached. Edit: this may be caused by a conflict between natural wonders mods. I'm using a variety of them. The logs are not giving me anything definitive. But it just occurred to me that I haven't come across a single natural wonder.