SUMMARY
In setting > desktop effects  > "inactive setting" there is no option for multi-screen setups.
The object of this affect is to darken inactive windows.

The issue = when this effect is active one screen appears brighter than the other screen. So it obviously obscured the other screen as well

Now if you chose:
-  settings > Window Behavior > focus > "Focus follows mouse (mouse precedence)"
and
-  settings > Window Behavior > focus > "Separate screen focus" = activated

you get a combination that prevents you from restricting the "inactive setting" effect only to the screen you are working on.

And I don't want to deactivate this effect for obvious reasons.


OBSERVED RESULT
On two screens: one is bright && the other is noticeably darker (depending on the defined darkness level)

EXPECTED RESULT
Get all my screens as bright as the level predefined in the settings > energy saving > display and brightness

SOFTWARE/OS VERSIONS
Operating System: EndeavourOS 
KDE Plasma Version: 6.0.4
KDE Frameworks Version: 6.1.0
Qt Version: 6.7.0
Kernel Version: 6.8.6-arch1-1 (64-bit)
Graphics Platform: Wayland

ADDITIONAL INFORMATION
ps: the "active screen follows mouse" option made it possible to counter this inconvenience by not affecting the brightness of the other screens.