Just to throw a spanner in all of the above theories and suggestions. I have noticed this frequently in my current project - always under or right next to track and always during the process of painting ground textures. Texturing is the last step in my route creation process so everything else, tracks and all scenery, is already in place.
These "artifacts", in my experience at least, only appear during the texturing process at a spot currently being textured when the texture "density" is still poor (i.e. during the first "coat" of paint). They immediately disappear when the full opacity of texture has been applied.
I have also noticed that texturing also seems to raise the ground level slightly in some areas (especially under splines) and I am not using any PBR textures, just the same ones I have used since TRS12 or even earlier. This may or may not be related.
Whether other factors are also at play here I do not know, but that is my theory and experience to throw into the mix.