Because Mp3tag removes automatically all single LineFeed characters from the tag-field UNSYCEDLYRICS within the MP4 file, when manipulating and saving this tag-field ...
(...??? maybe a Mp3tag bug ??? ...),
it seems so as if you have only one chance to modify the content of this tag-field, ...
by replacing each single LineFeed character with a CarriageReturn-LineFeed sequence.
Action: Format value
Field: UNSYNCEDLYRICS
Formatstring: $regexp(%UNSYNCEDLYRICS%,'\r\n|\r|\n','\r\n')
DD.20140912.1453.CEST