With gouraud or flat shading, setting specular power to 0 causes the surface to be rendered all black.
I ran into this strange issue in our Qt 5.15 and VTK 9.1 based FEA application on Intel UHD 770 graphics (and other native windows graphics). It does not occur however with the regular VTK openGL render window, nor does it happen when rendering through mesa. It can reproduced with Paraview 5.10.1 too (create a shape, e.g. from a cone source, and slide the specular power slider to zero).