Perhaps this is an explanation:
Here in this reddit thread they show how to create a wrapper (for google chrome, but maybe it could be adapted for other chromium browsers) to prevent that the fontconfig makes problems again.
https://www.reddit.com/r/Kubuntu/comments/1wo84kz/how_to_fix_crashes_when_googlechrome_corrupts/
KDE Plasma users seem to have even bigger issues than gnome users with crashing the shell because of chromium based browsers.