Created attachment 169865 [details]
Screenshot illustrating the focusing problem, Firefox and even a contextual menu of the same app is rendered behind a opened window of Dolphin

SUMMARY
When using KDE's Activities (this bug never occurs if I don't use this feature), after some times the window that is supposed to be focused will rendered *behind* everything else. Contextual menus are also affected (both menu created by the app (like the ones created by a GTK application, like Firefox) or menu created by Plasma/Kwin (ex. ALT+F3 menu)).

I would like to say that the problem will systematically affect all window, on every workspace, no matter what… but I'm afraid the bug is a little more vicious than that. But it will definitely happens with new window once KWin starts to become dumb, and will not only affect the window that you already created before switching to an another activity.

A little note to precise that if you have created numbers of activities inside your Plasma desktop, the bug will not happen if you don't switch to them at any point since your desktop's startup.

Manually restarting plasmashell ( `killall plasmashell && kstart plasmashell` ) is also not a workaround, I sincerely believe that this is a KWin problem.

I don't have any other hardware than this one using an integrated UHD 600 graphic driver so I cannot say if this is reproducible on an hardware with a NVIDIA or AMD GPU, sorry for that.
BUT I will precise later if this problem also happens on a Plasma X11 session, I did not think about trying before starting to write this bug report and I don't wanna rewrite it from scratch.

STEPS TO REPRODUCE
1. Create some activities, at least one
2. Do you stuff on both activities for some times
3. After some times, create a new window inside a previously opened activity, on a workspace that already contain at least one window
4. Profit

OBSERVED RESULT
On a "multi-window inside the same workspace" scenario, the focused window will show-up behind every other windows

EXPECTED RESULT
I'm going to take the developers for dummies if I describe the expected result.

SOFTWARE/OS VERSIONS
Operating System: EndeavourOS 
KDE Plasma Version: 6.0.5
KDE Frameworks Version: 6.2.0
Qt Version: 6.7.1
Kernel Version: 6.9.2-arch1-1 (64-bit)
Graphics Platform: Wayland
Processors: 2 × Intel® Celeron® N4000 CPU @ 1.10GHz
Memory: 15.4 Gio of RAM
Graphics Processor: Mesa Intel® UHD Graphics 600

ADDITIONAL INFORMATION

Bug also observed on OpenSUSE Tumbleweed, not an "ArchLinux Shenanigan".