Before assuming something is broken, check whether a nearby Weight slider or enabling toggle is set to 0 or disabled. Many sections in Emission Pro — such as the color Filter controls in Artistic Mode or Use Internal BSDF in Surface — are gated behind a master Weight or toggle, and the parameters below them stay invisible in their effect until that control is turned on or raised above 0.

There is no direct fix for this — it's a limitation of Cycles itself, which does not allow the volume contribution of a light object to be modified independently. A practical workaround is to duplicate the Spot light: on one copy, disable Volumetric visibility and use it only for surface lighting, taking advantage of properties like Radius and the spot cone angle; on the other copy, keep only the volume contribution active with its own color. This gives full independent control over surface lighting and volumetric contribution, using two lights instead of one.

Keep parameters within their default range whenever possible. Most sliders can be pushed manually beyond their indicated limits, but doing so — especially with color and contrast-related controls — can introduce unexpected visual artifacts. Use extreme values with parsimony, and treat them as a controlled exception rather than a starting point.