Hello,
I tried both, LibreOffice fresh 6.2.5-1 and dev 6.3.0.2-1, and they don't load the interface fully. As soon as I click them, they freeze. LibreOffice 6.1.6-2 works normally. I attached a screenshot.
Ok, problem "solved": The newer versions of LibO are trying to load the Qt5 interface plugin instead of Gtk3. Setting the environment variable to SAL_USE_VCLPLUGIN=gtk3 is a workaround until the bug wont be gone.
id never had guessed that