Yes, probably that was a bug. I tested the file and looks correct. I presume you just need to update Rhino.
As Kyle Houchens explains the best way,
Or workaround is to apply a regular wood material and then add an end-grain decal on the cut faces.
Remember to make first the UV mapping using Unwrap and selecting the edges. Or apply box mapping with caps.
I spent quite a bit of time trying to create a procedural wood SBSAR in Substance Designer specifically for this problem, but I couldn’t make it fully self-contained. As a workaround, I used Substance Painter to generate the final textures instead.
(Result: Wooden Car 5° - #3 by AlanMattano)
It would be very useful if a Substance material could handle this type of wood automatically.
The main limitation is that a proper procedural wood material needs TriPlanar projection to project separate textures for the side grain and the end grain.
The TriPlanar node requires Position and World Space Normal maps as inputs. These can be exposed as image inputs on the SBSAR. However, they must first be baked from the mesh. Since an SBSAR graph cannot access the mesh geometry directly, it cannot generate these maps itself. They must therefore be baked externally (for example in Substance Painter) and then supplied to the material.
I even experimented with embedding the baking process inside the material, but SBSAR graphs only accept inputs such as bitmaps, floats, colors, etc.—they cannot receive the mesh itself. As far as I know, the only workflow is to bake the Position and World Space Normal maps externally and then feed them into the Substance material.
In the end, I found it much simpler to use Substance Painter, bake the mesh maps there, and use TriPlanar projection to generate the final textures.