A few years ago, I ran Fedora w/ KDE Plasma. What _seemed_ to be random is the GTK fonts throughout the UI changed to a mono-spaced font regardless of what the control panel showed.
It "resolved itself" within a few weeks.
I don't know if it was an update that caused and subsequently resolved the issue, or something else I did.
This was just one of many hair-pulling examples I ran into across KDE Plasma and Gnome on Fedora.
I have different frustrations with both Windows and macOS.