SUMMARY
If you open the Desktop & Wallpapers setting for a given desktop and change a setting, such as the current Wallpaper or Right Click action for the Desktop (any setting reproduces the behaviour, these are just examples), going back to the defaults will not register as having unchanged settings. This is inconsistent with the rest of Plasma, where changing a setting but going back to the existing settings before the settings were changed will correctly register that no settings were changed (the "Apply" button will be disabled, and closing the window won't give warnings about unchanged settings).

As a simple and possibly common example, select a new wallpaper, and then click on the previously selected wallpaper (i.e. the current one), this does not count as an unchanged setting. The "Apply" button is still active, and attempting to close the window will give you a warning that unchanged settings will not be applied. I note this as a common example as I originally thought this only applied to Wallpapers until I was writing this bug report and tried other settings, and discovered it applies to all settings under "Desktop and Wallpaper"

STEPS TO REPRODUCE
1. Open "Desktop and Wallpaper" settings.
2. Change a setting, such as the current Wallpaper, but any setting will do (including Mouse Actions settings).
3. This marks the settings as changed, so the "Apply" button will be enabled and attempting to close the window will warn that there are unsaved settings. This is normal so far and not a bug.
3. Change the setting back to what it was when you opened the window, i.e. if you changed the wallpaper, go back to the previous Wallpaper
4. The "Apply" button is still enabled although the settings have been reverted back manually, and attempting to close the window will warn that there are unsaved settings. 

OBSERVED RESULT
Changing "Desktop and Wallpaper" settings and then changing them back while the window is open does not register that settings are unchanged. Attempting to close the window will result in an incorrect warning that settings will not be saved, although there are no changed settings to apply. This is inconsistent with the rest of the Plasma Desktop which will correctly understand that going back to the current settings means there are no unapplied settings.

EXPECTED RESULT
The "Desktop and Wallpaper" settings should understand that going back to the existing settings without applying them means that there are no unchanged settings. This should hopefully be feasible as other parts of Plasma are able to do this.

SOFTWARE/OS VERSIONS 
Linux/KDE Plasma: 6.9.7 Linux Zen
KDE Plasma Version: 6.1.2
KDE Frameworks Version: 6.3.0
Qt Version: 6.7.2

ADDITIONAL INFORMATION
I don't know when this began happening, it could have happened in Plasma 5 as well.