On my dual monitor system with Kubuntu 21.04, I have enabled several window effects like transparency or move inactive windows behind the active one.
The right monitor is the main one.
I also have Kodi 19.1 installed and the left monitor is the one, where Kodi is running usually.
With this setup, I can work on my computer on the right monitor and watch TV on the left monitor at the same time.

Now, I noticed, when Kodi is running, I'm loosing all my window effects, and the font size of the tray clock is getting a bit bigger.
After Kodi is closed, everything is back to normal again.

Is this meant to be like that or is there anything I can do to keep the effects working on the right monitor, even with Kodi running on the left monitor?

Obviously Kodi is disabling KWIN .
They do it for a reason they have there own 3d composite
Have you noticed they have their own compositor for that program.
I would not worry.
You will see more of this as xorg-xplasma xwayland and pure wayland
are implemented. Nothing to worry about.
last time I trouble shot that program it pretty much is headless and takes over your display.
If you have a discrete graphics card like nvidia and or AMD.
it will still happen.

Ok, I see, but is there any chance to enable Kodi's own composite only for the monitor, where Kodi is running on?