I'm having the same thing on the Inland Sea map. The river originates one tile down from the northern most point and all that shows is a strange pool of water at the originating vertex.
I'm not sure how to link the image so hopefully this works: http://steamcommunity.com/sharedfiles/filedetails/?id=794569951
I'm not sure how to link the image so hopefully this works: http://steamcommunity.com/sharedfiles/filedetails/?id=794569951