Hi,
The fact that box mode is also applied for Nurbs objects is known as ALS-9084. It will be solved very soon, by offering a switch to only affect subd's.
But you can already work around this by the following trick: diagnostic shaders either work globally, when nothing is selected (your issue), or on the selected objects, if some are selected when the diagnostic shader is activated. So you could select your subd objects before activating box mode, and you should be good.
Workaround for your Nurbs not being shaded: select them, and give them a diagnostic shader.
Why this happen in general: when activating the diagnostic shader, the hardware shade is deactivated. This is a limitation to the used architecture.
Hope this helps?
GG
Michael G.-G.
Manager Quality Assurance Automotive