The terminator artifact is there because the shading normal is different from the geometry normal.
Lux is asked to do the impossible by rendering something flat but making it look as if it were curved. It's doing its best to approximate that by using Phong interpolated normals (which are far from being physically correct), and in most cases it works, but in cases like this it shows up at the shadow terminator.

(Which is why in my opinion, all the "physically correct" renderers are false advertising: polygons are flat, and if you were to look at a polygonal model in real life, it would appear faceted, not smooth. When a renderer makes flat polygons look smooth, in my book that's make-belief and not physics.)