5.1ch DTS Express is recognised as 2.0ch

[macdavis]

The attachment is a 5.1ch DTS Express file. It is wrongly recognised as 2.0ch.

ffmpeg -i /Volumes/RamDisk/DTS_EXPRESS_256_5.1.dtshd -f null -
ffmpeg version 3.3.git Copyright (c) 2000-2017 the FFmpeg developers
  built with Apple LLVM version 8.1.0 (clang-802.0.42)
  configuration: --optflags=-Ofast --prefix=/usr/local --extra-cflags='-march=haswell -mtune=intel -m64 -minline-all-stringops -fomit-frame-pointer -fforce-addr -finline-functions -funit-at-a-time -funroll-loops -ffunction-sections -fdata-sections -Ofast -pipe -fomit-frame-pointer -ftree-vectorize' --target-os=darwin --arch=x86_64 --cpu=haswell --disable-runtime-cpudetect --disable-debug --disable-opencl --disable-avdevice --disable-ffplay --disable-ffprobe --disable-postproc --disable-ffserver --disable-devices --disable-avresample --disable-indev=qtkit --disable-doc --enable-pthreads --enable-version3 --enable-hardcoded-tables --enable-gpl --enable-nonfree
  libavutil      55. 69.100 / 55. 69.100
  libavcodec     57.102.100 / 57.102.100
  libavformat    57. 76.100 / 57. 76.100
  libavfilter     6. 95.100 /  6. 95.100
  libswscale      4.  7.101 /  4.  7.101
  libswresample   2.  8.100 /  2.  8.100
[dca @ 0x7f8539808600] Embedded LBR stereo downmix is not implemented. Update your FFmpeg version to the newest one from Git. If the problem still occurs, it means that your file has a feature which has not been implemented.
Input #0, dtshd, from '/Volumes/RamDisk/DTS_EXPRESS_256_5.1.dtshd':
  Metadata:
    fileinfo        : 	
                    : 
  Duration: 00:00:18.86, bitrate: 255 kb/s
    Stream #0:0: Audio: dts (DTS Express), 48000 Hz, stereo, fltp, 251 kb/s
Stream mapping:
  Stream #0:0 -> #0:0 (dts (dca) -> pcm_s16le (native))
Press [q] to stop, [?] for help
[dca @ 0x7f853900fe00] Embedded LBR stereo downmix is not implemented. Update your FFmpeg version to the newest one from Git. If the problem still occurs, it means that your file has a feature which has not been implemented.
Output #0, null, to 'pipe:':
  Metadata:
    fileinfo        : 	
                    : 
    encoder         : Lavf57.76.100
    Stream #0:0: Audio: pcm_s16le, 48000 Hz, stereo, s16, 1536 kb/s
    Metadata:
      encoder         : Lavc57.102.100 pcm_s16le
size=N/A time=00:00:18.85 bitrate=N/A speed= 631x    
video:0kB audio:3536kB subtitle:0kB other streams:0kB global headers:0kB muxing overhead: unknown