SUMMARY

In Task Switcher System Settings, the "Defaults" button is disabled despite changing shortcuts options (if no other option have been changed, e.g. "Filter windows by" options).
If some other options are changed then the "Defaults" button gets enabled and pressing it will reset all settings, including shortcuts, but the new default shortcut will not be updated onscreen.
Closing Task Switcher settings and coming back to it will than update shortcuts onscreen.
The "Reset" and "Apply" buttons are also affected by (or correlate with) the "Defaults" button malfunctioning.


STEPS TO REPRODUCE
1. Open System Settings, Window Management, Task Switcher
2. Press the "Defaults" button, if enabled. Than the button gets disabled.
3. Change shortcuts to new ones
   ("Defaults", "Reset" and "Apply" buttons stays disabled)
4. Change any other option (e.g. "Filter windows by" options)
   (Then "Defaults", "Reset" and "Apply" buttons gets enabled)
5. Press the "Defaults" button.
   (Any modified option will be reset to default, but shortcuts will not be updated onscreen)
6. Close System Settings
7. Reopen System Settings, Window Management, Task Switcher
   (Shortcuts are now updated onscreen)


OBSERVED RESULT
- "Defaults", "Reset" and "Apply" buttons stays disabled despite shortcut options changes.
- "Defaults" button will change shortcuts to defaults without updating it onscreen.


EXPECTED RESULT
- "Defaults", "Reset" and "Apply" buttons should be enabled after any shortcut options changing.
- "Defaults" button should change shortcuts to defaults updating it also onscreen.


SOFTWARE/OS VERSIONS
- Linux: 5.4.94-1-lts
- KDE Plasma Version:5.20.5 
- KDE Frameworks Version: 5.78.0
- Qt Version: 5.15.2


ADDITIONAL INFORMATION

- The same behavior occurs over "Main" and "Alternative" tabs.