When DP, the Plex app tells the video player 1 of 2 things. 1 - When you don’t manually select an audio track, “here’s a file, do what you want”. 2 - When you do specify a track, “here’s a file, play this video track and this audio track”. On the Roku, when it receives 1, it can override these and decide “I’ll play this audio instead since that is what my setting tells me to do”. Exoplayer in Android doesn’t have that override so when it gets 1, it always chooses the first compatible audio track. To use a “preferred track” the app would have to always use option 2, then you have the issue Cayars mentions. Another issue is some players don’t like option 2, they only listen to 1, which means the file has to be remuxed instead of DP anytime the preferred track is not first. As you know, DP is preferred as it provides a smoother experience.
Then throw in automatic bitrate adjustment and hardware acceleration, the potential scenarios go up. So there is a lot to consider before this could be implemented to all clients.
