In MacOS when an app is in the dock there often is a contextual menu list (right-click or control click) of recently opened documents, from which one can be selected to open the app with.  LibreOffice does have such a list when it is in the dock.  Unfortunately, it looks like the list contains every single document that has ever been opened (up to presumably some maximum.)  That list should be cleared whenever the "recent documents" list in the LO File menu or "recent files" in the start center is cleared.  Attached are screen shots of the contextual menu on my machine at the moment and the LO file menu and start center file display.  The latter two are empty; the dock list is not but should be.