Ok, you have passthrough enabled in the app, but it looks like you have it disable on your TV.
08-28 08:46:45.938 i: [HdmiCaps] Capabilties changed (AudioCapabilities[maxChannelCount=2, supportedEncodings=[2, 5, 13]])
Plex only sees that stereo is supported so only stereo is sent to your TV. Check your TV’s audio settings.
