STEPS TO REPRODUCE
1. Go to System Settings > Window Management > Window Behavior > Window Actions
2. Set Left Click to "Activate and Raise" in the inner window actions
3. Try to change a short cut key and hit "Apply"

OBSERVED RESULT
To get this, you may have to restart System Settings.
This happens for me when trying to change a key binding in "Shortcuts".  When trying to hit "Apply" the click does nothing.  You can no longer navigate to any other system setting.  Upon closing the window I'm prompted to see if I want to apply the changes I made, which works.

EXPECTED RESULT
The the "Apply" button would work properly

SOFTWARE/OS VERSIONS
Linux/KDE Plasma: Kubuntu 22.04
KDE Plasma Version:  5.24.4
KDE Frameworks Version: 5.92.0
Qt Version: 5.15.3

ADDITIONAL INFORMATION
This was a few hours of trying to figure out what went wrong with KDE.  By changing the Window Action setting back to "Activate, raise and pass click" everything works as expected with the System Settings.

I think this problem with hitting Apply was on some other forms.  I didn't take note of where else I saw this though.  At the time I was still early in working on this, and didn't realize it was a system setting problem.