I've encountered similar issues with various content on Telus Optik. I got tired of switching between Sorround and Stereo in the audio settings and just run in Stereo. I'm not sure if the issue is Telus or the source content. On the plus side it's fairly easy to toggle between the two audio settings.
Actually all 3 are digital formats. Stereo is 2 Channel PCM.
The issue is not all content is encoded in a format other than stereo. The audio settings state something to the effect that "if you are not getting sound to switch to Stereo". Most content works with any of the settings but I had to do this enough times that I ultimately decided to leave the setting on Stereo.
If there was a way to quickly change between the three settings I might be more inclined to use something other than Stereo.
Dolby Surround or Dolby Digital is similar to but not the same as Dolby Digital Plus.
Dolby Digital Plus provides up to twice the efficiency of Dolby Digital while adding new features like 7.1-ch audio, support for descriptive video services, and support for Dolby Atmos.