I don't get that behavior here. Clay mode or not the light and the lighting renders correctly. Note that if one's light is one sided geometry, only the positive normal side emits light and looks bright in the monitor and reflections. Also, in R20 with clay render mode, there is an Octane bug in which the Live Viewer gets stuck showing just one or the other. That is only in the Live Viewer though. Any render to picture viewer will use and render the correct mode, clay, color, whatever. Also, after rendering in the Live Viewer, the correct mode will again work in the Live Viewer.