The problem with this bug is that they need to add another end tile to the rivers for land endings, which they don't have (look at the xml for rivers and you can see it). They have beach endings and cliff endings, but not land endings. They need a lake tile and I would mod it in myself if I thought the game would see it, but right now I don't think it can.
I think that adding the third river end would solve the redering problem here, but I can't say for sure what monsters this would bring into the mix.
