Viewport shading 'constant' = 'no lighting' - bug?

I set up a new viewport display mode with ‘Lighting Method: no lighting’, to display only flat material colors.
This works for normal Rhino objects, but not for VA objects.

The grey slabs in this screenshot are surfaces (exploded VA objects), and are immediately displayed correctly:

Is this a VA bug, or am I overlooking something? Are there VA shading settings?