Holes in mesh generated by HB Shade component

When entering a mesh in the HB Shade (1.6.0) component in Rhino 7 there are lots of holes in the output shading geometry. Oddly, the mesh has 4514 faces but 5089 shading surfaces are generated. No errors are output from the HB Shade component.

What might be error? Alternatively, what might be a better way of generating the ground mesh as a shading object?

holes_in_shading_mesh.gh (155.1 KB)

I just realised it works if I triangulate the mesh first. But ideally the HB Shade component should warn me about this…