This happened around the time we switched to more physically correct shading equations.
I think previously we were computing diffuse lighting per-vertex and interpolating the result across pixels. With that change, we started computing diffuse lighting per-pixel, which is more accurate. I don’t think we knew about this case when we did that.
The issue in this thread, however, seems to be caused by a somewhat quirky feature we’ve had for many many years. We’re applying a directional fill light from the opposite direction as the sun/moon light. It’s illuminating the bottom left side of the noob’s face in your screenshots and is the reason why you see that thin slice on the noob’s shoulder that’s actually darker than the part of the arm facing away from the sun. This weirdness was hidden when we were interpolating diffuse across vertices. There’s not much reason to keep that quirk around anymore, so we plan to remove it later this year. That should make things look a little better in the noob case.
Btw there is no way that green hat is going to age well. The normal vectors of the mesh do not match the geometry well at all.