hit F9 ?!? 
I feel ashamed. The need to recalculate the fields simply slipped my mind. 
Of course, in some way Writer has to be told about the strings replacement. Maybe could my fault be partially justifiable if one consider the autoupdate behaviour with the uncompressed documents?

Anyway, the workaround to zip without compression is ok for me as LibreOffice actually will compress the document on first saving.