Environment CentOS 8.2, KDE Plasma version 5.18.4

As an example I mention hovering the mouse pointer over panel icons in kwin_x11.
Normally when you hover, a short description of the panel icon (or menu item) pops up.
Most of the time, these popup texts/boxes are graphically damaged (scrambled?). But not all of them.
It looks like it depends where they are located in the panel (eventually separated by so-called spacers).

Anyone an idea what might causing this?
rgds,
-gw

Found the answer myself.
It turned out I had the following (default) setting.
System Settings > Hardware > Display and Monitor > Compositor > Rendering Backend > OpenGL 2.0
Changing the Rendering backend into XRender > Apply solved my problem.