This is a longstanding issue that seems to have been mostly resolved in previous versions, but I've noticed it again on a new model I've made today. This model replaces upside-down stairs, but it seems to occur regardless of the block as it is related to the model and smooth lighting.

With smooth lighting enabled, elements/cubes of custom block models rotated 'diagonally' (22.5, 45, -22.5, -45) are inappropriately shaded. There was a similar issue with these shadows being completely black in the past, but here they are more subtle.

In the enclosed images, I have the stairs/model placed in a 3-block high 'arch'.

In Temp.jpg, you can see a stop in the shadow's smoothness/ramp where the top of the model is shaded more heavily than the bottom.

It's more obvious on the blocks in the middle than on the edge (e.g the rightmost column), again because it seems to be an issue with smooth lighting and diagonal model elements.

In Temp02.jpg, smooth lighting is turned off. The model has shading and ambient occlusion enabled in its .json.

Temp03.jpg is using a shader and is probably outside the realm of fixing for Mojang, but also highlights the shading issue. In this case the broken smooth lighting is flipped, and shaded unevenly toward the bottom of the block instead of the top. This matches the issue without shaders enabled, as seen in Temp05.jpg.

Temp04.jpg is only for reference, and is of the model itself in a model editor.

 Temp05.jpg shows the model with ambient occlusion and shading switched off in the model .json, but smooth lighting turned on. The blocks with the black shadows are under full blocks, where the blocks without them are under transparent blocks.