The flatpak version of Libreoffice only installs a single launcher. It would be nice if it installed separate launchers for Writer, Calc, etc, like it happens on other distributions.
It did in the past. It has been changed because of https://bugzilla.gnome.org/show_bug.cgi?id=771767 .
...and fixed again since LO 5.4.2 with <https://cgit.freedesktop.org/libreoffice/core/commit/?id=35fc8885760030cb78b856a23ad02f5148268296> "Include all .desktop files in the Flatpak"