It can't decode DTS-ES or Dolby EX (standard limitation of disc players), so when encountering those tracks it decodes the basic 5.1 track. There should be no need to downmix to 5.1 because it will only see a 5.1 track.

When we start talking about Blu-ray, things change. DTS-ES tracks are not common on Blu, and DTS-HD is a whole different animal in many respects. A 6.1 DTS-HD track would be decoded internally to 6.1 and downmixed to 5.1 in your case.