by default libreoffice latest flatpak is not installed (flathub is not enabled in app center).
you should visit flathub.org and download the flatpakref file for libreoffice (or firefox etc) and open it will ask "do you really want to instsall" after you install at least 1 app like that then app center will show all apps from flathub.org which you can install whichever you like.