In my opinion, the tag shading_mismatch should not exist, or be used a lot more often. In many artworks, even when the shading is way realistic, the face ends looking kinda flat, unless the art style is so realistic, that it changes the shape of the head to a realistic head shape and lets add realistic shading to the face. Because if you haven't noticed, in many anime art styles (like this pic for example) the head has an unrealistic shape, that changes conveniently depending on the angle, and when the face can be seen up front, is actually flat.
Not all art styles are like this as I said before, basically, those that look less "anime" if that make sense, adds "correct" shading to the face.
Normally in anime, the most realistic shades on a face can be seen on a 3/4 angle, as it's easier to make shapes in that angle, like the nose, for example.

Agreed. Was trying to figure out what the tag meant by looking at the pictures (a whole whopping 24) without checking the definition. Nothing bad or obvious jumped out at me.