I tested the residual violet/UV thing for one of my enlarging papers in this thread (skip to post #56 for the test but there is some good discussion beforehand on whether or not there is even significant UV in the system):
Even if this issue existed with Marco's paper, it would affect visual focusing the same way as with a grain magnifier. Marco, if you want to rule the supposed/possible residual UV thing out, repeat your test at the lowest contrast setting (ie max yellow) and see if there is any difference in the results. Try it at maximum contrast for comparison.
Based on everything said so far, if you are certain there is no drift in your lens stage I'd say there must be something wrong with the magnifiers, as strange as this seems.
2mm is significant at the negative plane, much less so at the paper plane.
It is a strange and somewhat alarming situation. Interestingly, I have consistently found when I focus by eye and then reach for the grain magnifier, the grain is already sharp. It surprises me every time. I still always use the grain magnifier but a few times I've tried an actual print both ways and couldn't see a difference. I've never seen the visually focused print look sharper the way Marco has though. Very puzzling.