Seems to be related to how ffmpeg is detecting those tracks. Apparently there’s a special encoding of 7.1 formats to make them compatible w/ 5.1 layouts where ffmpeg will identify the track as 5.1 / 6 channel format.
There’s a longer thread about the technical details.
From doing a quick scan over the thread it seems ffmpeg has released a newer version that should handle such tracks better – no ETA as for when Plex will incorporate that version
