Yes the Hahnemuhe FAB is on 2-ply mat (not mounted but just temporarily lightly held in place with double sided tape- a standard way to use front feed and avoid pizza wheel marks). I tried with Wider platen gap and still get the same issue. It turns out that it is a very faint manufacturing defect in the Hahnemule paper. It can just be seen with side lighting as a subtle gloss differential on the remaining unprinted paper in the box. I have contacted Hahnemuhle to let them know.
Walker, does a Wider platen gap lead to less sharp prints, in general?