Ok, so I handbraked the file to a low complexity version and added a 2ch audio as well as leaving the 5.1 although the 5.1 as an AC3 (could not seem to get Handbrake to give me a AC MAIN output).

Still using the normal profile I tested the 2ch and it plays without transcode fine, when I select the 5.1 on the app and play it triggers a transcode and plays fine too.

Not sure if this suggests its the Main rather than LC format that doesn'nt get transcoded.

I am struggling to find any examples of or be able to create an AVC/ACC Main in 2ch to test with.