Created attachment 167405 [details]
Example setup with two monitors, and the resulting kdialog notification.

SUMMARY
Using the shortcuts for "Switch to Screen to the Left/Right" does not work in Plasma 6. It says the window manager is configured to consider the screen with the mouse on it as the active one (it is not).

STEPS TO REPRODUCE
1. In Plasma Settings, go to Window Management > Window Behaviour, and enable separate screen focus.
2. Go to Keyboard > Shortcuts > Kwin, and set a shortcut for "Switch to Screen to the Left/Right" (or Next/Previous Screen).
3. Press the shortcut.

OBSERVED RESULT
 A kdialog notification appears, saying:
The window manager is configured to consider the screen with the mouse on it as active one.
Therefore it is not possible to switch to a screen explicitly.

EXPECTED RESULT
The window in the second monitor (Kate in the example screenshot) should be activated.

SOFTWARE/OS VERSIONS
Kernel version: 6.7.9
KDE Plasma Version: 6.0.2
KDE Frameworks Version: 6.0.0
Qt Version: 6.6.2

ADDITIONAL INFORMATION
Tested on both X11 and Wayland with the same result.
I updated from Plasma 5.27, but this bug is also present on a fresh account with a default config (except for the changes in the steps to reproduce).