characters in math not printable

[tweh]

When I compile the following MWE with LuaLaTeX and print it from Adobe Acrobat the + and = isn’t printed, although visible on the screen. This doesn’t happen if

• using another viewer to print or
• compiling with XeLaTex or
• using different weights for math and text or
• removing the text mode a

% !TeX program = lualatex
\documentclass{scrreprt}

\usepackage{fontspec}
   \setmainfont{Fira Sans Light}
\usepackage{unicode-math}
   \setmathfont{Fira Sans Light}

\begin{document}
a
\( a+b=c \)
\end{document}

The last time I compiled (2017-07-16) I didn’t had this problem. So it might could be caused by the recent changes in LaTeX3.

[tweh]

Update
It works, when using Acrobat on Windows (don’t know the version number). On my macOS 10.12.5 with Acrobat Pro DC 2017.009.20058 the above described error occurs. However if I use the preflight tool with Potentielle Schriftprobleme beheben (I guess “Fix potential font errors”). It reports

• Schriftnamen eindeutig machen
  (“make font names distinct”)
• Fehlerhafte ToUnicode CMap Informationen in Schriften korrigieren
  (“fix erroneous ToUnicode CMap information in fonts“)

After this preflight and fix I can print the PDF without problems.

I guess the problem has something to do with Fira being embedded twice (as text and math font) …

[wspr]

Sorry, I don't think this is anything I can help with in unicode-math — this is more of an engine problem.

[tweh]

Hm … thanks anyway … do you know where I can report this bug then?