In the image you can see 4 roboports in the top right, and a couple others in the bottom left. These two areas actually form a logistics network together as is visible in the center of the image where the logistics areas connect. However there is not a single yellow line that connects the group of four with the rest of the network.
The circled roboport is the only one to connect with the logistics area of the group of four, removing this roboport created two separate networks. As is obvious in the image, there is a different line between two other roboports, preventing a line from the circled roboport to the group of four. So it seems like the algorithm doesn't recognise that this line is required and thus doesn't mind drawing the blocking line instead.
The problem is very easy to reproduce, you just place the relevant roboports in roughly the same formation; one roboport in the top right connecting to one roboport in the bottom left; with two other roboports connecting in the bottom left such that their connection line is inbetween the first two roboports. Here's another image with the problem isolated:
I'm not entirely sure whether this can be counted as an actual bug, or whether it's acceptable behavior from the algorithm. I didn't see anything about this mentioned in the known bugs or won't fix areas.
I've omitted any other information such as a savefile or logfile as these shouldn't be needed/relevant; the problem is clear from the above images and description.
