Long strip of world unroutable
I am currently created a modified version of Riverview. The problem is that in game a whole strip of my world is unroutable. By "unroutable" I mean that there is no go here interaction in this area, although sims can still walk and drive through this area to get to other routable areas, if that makes sense. Strangely, if the sim is on a horse, the ride here interaction does still appear. The ground floor of lots in this area is also unroutable, but not the upper floors. When I looked at the routing data, I saw that there is a long red line corresponding to the area that is unroutable. I have tried everything I can think of to fix this issue. I have deleted and replaced all the roads, lots and objects in this area twice. I have completely flattened and then rebuilt the area. I have of course checked that there is no non-routable paint there. Does anybody know what might be causing this issue or how I can fix it?
In the image attached, the unroutable area starts at the red line, and finishes at the blue line I have drawn.