Bug 457301

SUMMARY
If some new setting is applied in the fonts KCM, applications that are currently open start to use the (new) general font for things like menus and toolbars. The new menu/toolbar/... font settings will only take effect after the application is restarted.

STEPS TO REPRODUCE 
1. In system settings, set the general, menu and toolbar fonts; say we have Noto Sans Regular, Noto Sans Bold, Noto Sans Italic respectively.
2. Open a Qt application that have context menus and/or toolbars, say Dolphin. It should use the fonts above.
2. Keep Dolphin open. In system settings, change any setting and apply; for example, change the menu font to Monospace.
3. Right-click to open the context menu in Dolphin.

OBSERVED RESULT
In the last step, Dolphin uses the General font (Noto Sans Regular) instead of the Menu font (Monospace) for the right-click menu.

EXPECTED RESULT
Dolphin should use exactly the new font settings. (use Monospace for the menu)

SOFTWARE/OS VERSIONS
KDE Plasma Version: 5.25.3
KDE Frameworks Version: 5.96.0
Qt Version: 5.15.5

ADDITIONAL INFORMATION
I think this is a problem in the KDE Qt platform theme:

https://github.com/KDE/plasma-integration/blob/bc6f94f02709b4b2fc7c3da3149c330eedd39a16/src/platformtheme/kfontsettingsdata.cpp#L93

When new font settings are applied, lines 93-97 cause the existing application to use the general font as the default for all widgets.