See also: https://bugzilla.gnome.org/show_bug.cgi?id=673097
In some cases, applications put files into hidden/temporary directories. For LibreOffice's target user demographic, it does not make any sense to allow saving in those directories.
When the current file is in a hidden directory or /tmp/ or whatever, when saving LibreOffice should not pass a default directory as an argument to the filechooser (therefore letting the filechooser do its job at offering the user with a better default), or at least use a better default folder (I think LibreOffice suggests the XDG "Documents" folder as a default?).
*** This bug has been marked as a duplicate of bug 43895 ***