Lightroom Classic: luminosity range mask does not work Leica Monochrom DNG files.

I cannot seem to get the Lightroom  luminosity range mask to refine a selection working with Leica Monochrom DNG files.  The Select —> Color Range works in Photoshop.   I can make a selection in Lightroom using the brush/gradient tools but I cannot refine them with the Luminosity Sliders.