Hi,
I have two bullseye systems with libreoffice from backports which show the issue. Both run plasma-desktop from bullseye. If I open a second libreoffice document, the window is visible as a control panel entry of plasma, but the window itself is not visible on the screen. The window appears on the screen if I click on maximize on the control panel entry. It seems like the window is coming from the top left of the window of the first libreoffice document,
What happens if you remove libreoffice-kde5?Could also be some similar thing as https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1019423?
Regards, Rene