I used the Premium Luster Photo Paper 260 setting and the premium semigloss 250 and both produced the same unusual result. On my old 4800 the same settings produced good results.
I do not think it is a dithering problem - are you able to scan with high res, eg. 3200 dpi both samples?
It could be a printer failure, or bad fraction of paper (it happend to me once, not with HM, but another German company, that one pack had a paper behaving very different than usual - like overinking).
It could be also accidental increase of color increase tab in the driver... nothing more comes to my mind.