All of the faces of the polygons created bybar3d are oriented (using a right-hand rule on the vertex order) in the positive axis directions. As a result, the shading used byshade=True is incorrect, as it shades both the front and back faces with the same brightness.

Rotating thehist3d demo, we can see the left and right faces of the bars are the same color:

The fix is fairly straightforward, thankfully