I often use the face weighted normals modifier on relatively low res geo, and I’ve been experiencing weird black shading artifacts around the edges of meshes. I googled it, and it seems that this is a known limitation with prorender that C4D users also deal with. Here is an example of a C4D user experiencing the exact same render defects:
The best way I can describe the problem is that prorender doesn't seem to calculate vertex normals correctly which leads to ugly black edges that almost look like a toon shader.
I've attached a very simple blender 2.8rc2 test scene. Platform is windows 10. The issue persists on 2 machines, both with nvidia gtx 1080 ti and latest studio drivers.