The Plex media server allows the default audio track to be set. However, Infuse doesn’t appear to recognize that setting and instead defaults to the first available audio track? In this case, it is a MKV file with DD 5.1 as the first track and Dolby TrueHD 7.1 for the second. I have the Plex setting as the second track which the Apple TV Plex player honours. (yes, unfortunately, Plex does transcode it to EAC3 which is one of the reasons I am using Infuse.)
Does the Infuse app have access to this setting when streaming from Plex?
Infuse has its own settings for audio and subtile track selections which can override what Plex has. However, these can be adjusted in Settings > Language.
One thing to note, when set to Auto, Infuse will honor the ‘default’ tag if present for any of your tracks. This means Infuse will prefer this track over other tracks, regardless of language. If you prefer to avoid this, you can adjust the setting from Auto to your desired language and Infuse will ignore the default tag.