Created attachment 154745 [details]
As you can see, the blur is only applied to a part of the window.

SUMMARY
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STEPS TO REPRODUCE
1. Make sure you have blur enabled in Desktop Effects
2. Open an application that uses transparency and blur (in this case, it's the Konsole app)
3. Maximize the window, put it in window mode and activate it again
4. The blur is only applied to a part of the window
5. If you open the app maximized, the blur works properly.

OBSERVED RESULT
The blur is only applied to a part of the window when you put it in window mode and maximize it again. Disabling the compositor and re-enabling it makes the blur work again until you reproduce the 5 steps. 

EXPECTED RESULT
The blur would be applied to the whole window after maximizing the app.

SOFTWARE/OS VERSIONS
Linux/KDE Plasma: EndeavouOs, X11
(available in About System)
KDE Plasma Version: 5.26.4
KDE Frameworks Version: 5.101.0
Qt Version: 5.15.7

ADDITIONAL INFORMATION
On Wayland, the app flickers when maximized, but after that it works. This bug only applies to X11. Also, tiling the window or moving it in windowed mode works. It's only the maximizing which causes the problem. The only third-party Kwin effect is lightlyshaders-git from the AUR, and disabling it didn't help either.

Specs (I have a Thinkpad E14):
- i3-10110U
- RAM: 8GB DDR4 2667 MHz
- GPU: Intel UHD Graphics
- GPU driver: mesa 22.3.1-1