The problem is definitely not with LibreOffice. I have experienced the same issue with virt-manager. More annoyingly, once you try to select the menu, you cannot seemingly close the window easily either.
As a data point, the bug seemingly cropped up after the first introduction of KDE 6.3. My desktop did not have this issue until I updated today. The last update before that was on the 17th. My laptop first experienced this issue after updating on Monday evening.