Been using sketchfab to show house models for a while and lights seemed to work as expected.
Now, when I light anything with a single direction light there is a problem. When the light points directly at a surface it does nor illuminate it. Rotate the light slowly 90degrees away from the surface and the surface becomes illuminated and then gets darkers as the light turns away from the surface which is the correct behaviour. However, if you rotate the light away from the surface in the other direction then the surface stays dark.
On this model rotate the light up or right and the lighting works (darkens gradually as lights rotate away). Rotating down or left and the lighting breaks immediately.