ffmpeg – no audio when encoding 5.1 ch audio to opus

when i was encoding a dts 5.1 ch C L R Ls Rs LFE audio, file using ffmpeg via libopus , i found that the resulting file did not play in vlc, due to the channel layout being rewritten to L R C Lb Rb LFE, whereas the original file could. however if downmixed to stereo, there would be sound. the attached link contains a copy of the song file which i reported to have no audio when encoded into opus format in 5.1 ch mode
command used:

ffmpeg -i out.dts -c:a libopus -b:a 160k -af "channelmap=channel_layout=5.1" out.opus