the UI font (for mscore screenshots) is tricky

‣ MuseScore loads DejaVu Sans by default, which doesn’t have the
  font embedding exception

⇒ switch to another UI font

$ sudo apt-get install qt5ct
$ QT_QPA_PLATFORMTHEME=qt5ct qt5ct
$ QT_QPA_PLATFORMTHEME=qt5ct musescore

We use Fira Sans now, which is under the SIL OFLv1.1 as almost all
other fonts, from texlive-fonts-extra (in Debian), by recommendation
of: https://thomasbyttebier.be/blog/the-best-ui-typeface-goes-unnoticed

LICENCE

@@ -54,6 +54,8 @@ embedding exceptions:
   optionally bold and/or italic (GPLv3 with font embedding exception)
   from MuseScore (embedded), for example scores and screenshots
 
+• Fira Sans (SIL OFLv1.1) may occur as screenshot UI font
+
 The subdirectory html/ contains a browsable export of the slides
 from the MagicPoint format sources, embedding the aforementioned
 fonts and the source images.