Dissolved solids in the lacquer may be drying up to a very dark gray. That would definitely be the case if it's a matte lacquer, which would contain something like super fine powders to prevent the dried surface from being glossy.
For the Epson Cold and Hot Press papers, and also BC Pura, I have found that d-max lowers slightly when using extremely clear water based coatings, which necessarily dry to a gloss or satin finish. But matte finishes will always raise d-max from what it was in the bare print.
Don't know if the paper you are using is "micro-porous" but in general those types of papers are very poor candidates for coating, except with some of the very low solids solvent aerosol sprays.
