@dntaylor
Interesting.
I have played this file on both my HD51 and GBue4k via hdmi into an Onkyo tx nr626 (which supposedly supports 4K passthrough but mostly doesn't!! ... but supports all the audio formats and allows you to monitor input & output streams)
The ts file shows (mediaInfo):
1 video stream @ 1920 x 1080i25 which plays quite happily at whatever output mode I set.
1 audio stream AAC and another "menu" stream AAC. The 1st stream will produce good audio (English, 48.0 Khz, 2 channels, AAC (LATM) (LC) the other will not. So via your audio button on the Giga4K remote just check which audio stream you are playing.

If I don't change defaults both the HD51 and Giga4K play audio that comes into the Onkyo as PCM stereo.
On the Giga4K: (using menu/setup/audio & video/settings) ... using the audio button on remote doesn't seem to change anything
If I enable AAC downmix, nothing changes -> PCM stereo.
If I enable transcode to AC3 nothing changes -> PCM stereo.
If I enable transcode to DTS -> DTS 2 channel stereo

So I have no issue with the video stream and providing I select the "correct" audio stream that also works fine.

So based on this the GB4K appears to not be the issue in your environment as I would expect PCM 2 channel stereo on the optical............... I will try the optical into my amp later if I can find my optical leads!