I had the same problem.
There’s another thread here about graphicals problems with LibreOffice https://discourse.lubuntu.me/t/bug-in-the-libreoffice-graphical-interface/1790.
I agree that could be a qt5 issue.
By the way I solved with:
- purge all libreoffice packages
- reinstall them with
sudo apt install libreoffice libreoffice-l10n-it libreoffice-help-it --yes(change “it” with your own language).
Maybe sudo apt purge libreoffice-qt5 could be enough, but when I did that I didn’t know the qt5’s problem