You can actually see the lines between overmaps (large rectangular areas in the world map). The forests cut off between two overmaps. By watching the coordinates in the lower right corner you can detect where the overmap borders go. You could also use the debug functions to reveal map, which will reveal the current overmap. Then use debug teleport to travel just outside the overmap, and use the ‘reveal map’ command again, and take a closer look at the border between the two overmaps.
What I think you have there is crossing (corners) of four overmaps, just bit west of the FEMA camp.
Map generation isn’t perfect. The more distance you set between cities, the more disjointed the cities become. With high enough settings, you will eventually see roads end and there’s just roadless land between (far-apart) cities, the landscape being littered with farms, labs and such, with no roads leading to them.
Edit: No clue what the grey dots are.