Rhia, if you look closely at ghostship2's screencap, there is a modification from the Redfilter node redirected to the Subsurface channel.  I don't recall if that node is completely disconnected from the Clearcoat or shares a split secondary connection. In any case, the Redfilter node overrides the default Subscatter values, driving those values lower if the subsequent map is dark enough. I wonder if there is a need to insert a gamma node to properly bring the gamma value of the grayscale map to 1.0.