The pathfinder in TTD isn't the best, sure it does things it shouldn't do,
the best way is to avoid irregular platform entrances for lines....
To the description of nonningstone.png, it could be aswell the steep slope there, there is enough room for an additonal platform, try to experiment with it a bit.
I am not quite sure for what you created that narfingburg roundabout, it's actually not RCT you are playing

I will ask Josef if he has ideas why the pathfinder goes wrong, until now
try to avoid haveing to complicate layouts. The pathfinder for was not changed so the same flaws are in as before...