Server Version#: 1.19.1.2630
Player Version#: 1.6.5.1097
This issue has been posted a few times over the years, such as here, here, and here.
In that last thread, from 2018, @OttoKerner mentions raising it as a bug, so I was wondering what the status of this issue was since its still affecting many of my older films with main mono audio tracks and supplementary audio tracks with more than one channel.
As mentioned by some of the previous reporters, all audio tracks can be of the same language, and the default flag can be set on the main (and first) audio track, but it will still be skipped over in favour of the first multi-channel track, somewhat contrary to the Matroska specification.
From what I understand, Plex seems to largely ignore the default flag on tracks, though it seems like here the information that flag provides would be a good hint to the player about which track should be preferred when there are multiple valid choices (such as multiple tracks of the same language), and which I think is the general purpose of that flag.