As the summary says, when installing/removing multiple fonts to system the expected behaviour would be for fontinst to keep running after the first font has been installed or removed, instead it quits which causes a bunch of issues. fontinst uses KJob to authorize and internally KJob uses a QEventLoopLocker which causes the main event loop to quit when it's done.
I'm not entirely sure why the event loop locker is enabled by default for KJob, but the patch I have attached works around this by completely disabling that functionality for fontinst. There's a timer that runs in the background which checks for any connected clients, so it will quit after a little while regardless.
See #379524 and #379324 at bugs.kde.org.