the thing is that soft selection channel is a vertex channel (not a map channel). only map channel can be used as vertex color display. except… yes. soft selection.
but max handles it specially.
so we have to copy float values of soft-selection channel to a map (point3) channel and colorize a float weight. as i remember there is a formula that converts soft-weight to red-to-blue gradient. (not exactly correct but it’s a half way of HUE in the HSV color representation)