It is arduous to right click on each font, open up KFontView and click "install font" when installing fonts, the other option is to do everything with idiosyncratic console commands sudo fc-cache -f -v. Reproducible: Always
I can confirm this. But I seem to remember it being possible in KDE 4? So is this a regression in the move to KF 5?
This is now a feature, it seems. However, there is a bug in the system-wide font installation that causes only two fonts to be installed at once, see https://bugs.kde.org/show_bug.cgi?id=349673