Windows 7, Mp3tag 2.73.
Steps to reproduce:
- Add UNSYNCEDLYRICS to Tag Panel (multiline tag).
- Open any .m4a file.
- Write multiple lines of any lyrics. (I've actually copied the lyrics from a file with proper Windows line-endings.)
- Open the same file in foobar2000, check in Properties, and see that all lines are displayed as a single one.
- Opening the file in a hex-editor shows that line-endings are written as a single '\r' (0D), instead of a proper "\r\n" (0D 0A).
The same lyrics written to a .flac file (with a mapping UNSYNCEDLYRICS -> LYRICS for VorbisComment) are saved properly.
It turns out that iTunes also writes Lyrics with '\r' line-endings, but I didn't find any information confirming that it's a standardized way to store multiline tags in an .m4a. Most Windows software cannot handle such multiline text. Since Mp3tag is mainly for Windows, it would make a lot more sense to use line-endings native to that platform, especially because other platforms handle Windows line-endings well.