[XeTeX] misplaced accents in printout only?

[Dominik Wujastyk]

A student of mine is preparing a PhD thesis with XeLaTeX.  He recently sent
me his​ draft as a PDF (xelatex -> dvipdfmx (20150315)).  I attach a page
of this, kkk.pdf, extracted from the thesis with pdftk.

The Sanskrit text in roman script contains a number of letters that have
under-dots, like ṣṭḥṃ etc.  He is typing this with a Unicode-aware editor,
SublimeText.

His PDF output from XeTeX displays on the screen just fine, using Okular or
Evince (Linux Mint 17.3 etc.).

But when I *print* his document on my HP LaserJet Pro 400 MFP printer, the
underdots have turned into overdots, and are shifted slightly
horizontally.  I attach a scan of the printed output, kkk-pdf-scan.pdf.

I haven't encountered anything quite like this before, and it baffles me.
I've tried outputting the PDF to PS and printing that.  Printing the PDF to
another PDF.  I've tried using Evince not Okular.  Always the same
problem.  Everything points to the printer or the printer driver.  But this
is not a hole-in-the-wall printer, and I'm using HP's own driver.  HP knows
how to interpret PostScript, surely.

The system details for the installed printer and driver are:

HP_LaserJet_400_MFP_M425dn
--------------------------
Type: Printer
Device URI: hp:/usb/HP_LaserJet_400_MFP_M425dn?serial=CNF8H3NM1D
PPD: /etc/cups/ppd/HP_LaserJet_400_MFP_M425dn.ppd
PPD Description: HP LaserJet 400 MFP M425 Postscript (recommended)
Printer Sending data to printer.Jet_400_MFP_M425dn is idle.  enabled since
Mon 18 Jan 2016 14:27:39 MST
Required plug-in status: Installed
Communication status: Good

I've run HP's diagnostics, and it is satisfied that the printer is properly
installed and everything's up to date.

Can anyone shed any light on this problem?

Dominik
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