I have searched on the web for over two days now, and probably have looked through most of the online documented scenarios and workarounds, but nothing worked for me so far.
Another possible issue might be that the meta values contain non US-ASCII characters. For me it helped to UrlEncode the values when adding them to the putRequest:
request.Metadata.Add(AmzMetaPrefix + "artist", HttpUtility.UrlEncode(song.Artist));
request.Metadata.Add(AmzMetaPrefix + "title", HttpUtility.UrlEncode(song.Title));