I have several templates that I use to create documents from a Base db. In OOo 3.3, printing a mail-merge creates editable documents that I can tweak if necessary. In LibreOffice 18.104.22.168, it creates documents filled with the fields that should have been replaced. The replacement text is in the field, but is not editable.
Please see pictures at https://picasaweb.google.com/112200146893522559516/MailMerge?authkey=Gv1sRgCInbnpW49KmULA