Rectangular Area Lights require extremely high values in order to illuminate Volumetric Fog and Volumetric Materials. This means they do not appear to match the output of other light types of identical intensity.
To confirm that volumetric fog works at all with Rect Lights, I changed the Intensity Units to Unitless, set the Intensity to max, then changed the unit back to Candalas. This set the Intensity value far higher than the maximum for Candalas and allowing the illumination of the fog to be seen.
Expected: Rect Light illuminates fog at about equal to the pointlight nearby, which shares the same Intensity
Result: Rect light does not appear to illuminate the fog
There's no existing public thread on this issue, so head over toAnswerHub just mention UE-61604 in the post.