If I’m not mistaken the problem is with GTK applications on the KDE desktop.
I have had this problem in the past because of my preferred font on KDE (Utopia). I was able to correct it by selecting a different font for GTK applications.
With the last KDE update, however, it appears to me that the module in “systemsettings5” for selecting which font to use for GTK applications is gone. I suppose this is progress.
For me the fix was to change my system font to something that works with GTK. I switched it to STIXGeneral but I’m sure there are others that will work.
I’m looking at GNOME/GTK Application Style. On my system it only gives me the option of changing among “styles” like Breeze, Adwaita, etc. There is no option just to change fonts that I can find. Although there used to be.