From LibreOffice flatpak site: “The LibreOffice Flatpak is using LibreOffice’s GTK3 backend.”
If you really want LibreOffice with QT, then maybe install it inside distrobox. The simplest way is to use BoxBuddy. I have written instructions for some other program, how to install program in distrobox using BoxBuddy, but it should be obvious how to use LibreOffice instead.