Normally, the W-E sides are darker and N-S are lighter.

When the model is rotated +/-22.5 degrees, both N-S and W-E become darker. When the model is rotated +/-45 degrees, N-S becomes darker and W-E suddenly becomes lighter.

It would be expected that either a certain side will always be the dark one, or that the one closest to W-E would be the darkest.

It could use the same shading method that is used in entities. Bedrock Edition has block model animations - if that's added in Java eventually, the shading would need to change smoothly when rotating. 

There are also issues when you disable ambient occlusion and/or shade in the model. The faces that stay in the same plane will be shaded normally, but the faces that change their plane will render dark.

I included a resource pack with the rotated models. Cobblestone is not rotated, and the planks are ordered based on rotation like this:

Y-axis: 22.5 | 45 | -22.5 | -45 | X-axis: 22.5 | 45 | -22.5 | -45