When using 3D modelling software I found that triangulation made more planes on one face of the shape than there actually was, for example in creating one hexagonal face I found that the process would create one rectangle and two triangles that would present two distinct lines running through the hexagonal face. The complete shape outline would be shown but it would have more closed faces that could for instance be, filled with colour or deleted.
Although I never printed any versions like this when exporting the file into a picture viewer those closed faces remained.
This presented a confusing position for me as novice as my starting point was hexagonal and to fill the sides of the shape triangles were necessary.
I can not for certain know the reasons for these lines on this face of the Multijet fusion black Nylon PA12 version. This was a polished version so I left in place. As it did return my thinking to a previous issue I had to overcome.