I only started a month ago, so not sure on the version this started on, but I have noticed these black ghost lines in the Midori map exclusively since I started playing.
On the earlier maps, there are also black smudges. These move with the camera and are not burnt into the terrain.
They do not appear on every level, but every one of the first 6, then on and off the ones after.
Video card: GTX 960
Image with red outlined around the general outer part.