No surround on Android TV but surround on smartphone

Hi guys,

Here's an issue I can't wrap my head around: I can't get surround sound from my Android TV with the Moonlight app!

The set up:

• Samsung Home Cinema HT-C5900 receiver
• Samsung TV UE32ES6100
• Connect TV SFR with Android TV Pie 9.7.17

The Connect TV player is provided by my ISP. There's no documentation whatsoever conserning the audio options which are as follows:

• Dolby sounds:
  • OFF
  • LINE (currently set on this)
  • RF
• Audio Mixing:
  • Toggled off, it reads: "support 7.1 channels through Dolby Audio playback"
  • Toggled on, it reads: "support 5.1 channels through Dolby Audio playback" (currently set on this)
• Audio digital format :
  • PCM
  • SPDIF
  • AUTO (currently set on this)
  • Manual (which opens up several toggles):
    • Dolby Atmos in Dolby Digital Plus
    • Dolby Digital
    • Dolby Digital Plus
    • DTS
    • DTS-HD
    • AAC
    • Dolby TrueHD
  • DTS DRC sound scale with options from 0 to 100% going 10 by 10 (currently set to 0)

The Connect TV player HDMI out is plugged to one of the HDMI in of the 5.1 receiver, and the HDMI out of the receiver is plugged to the HDMI ARC of the TV.
Additionally, I have the SPDIF of the Connect TV plugged to the SPDIF of the 5.1 receiver, so I can enable Dolby Pro Logic when the audio source is stereo (in that configuration, I unplug the HDMI from the receiver and put it on the TV directly).

I'm getting 5.1 surround sound on apps such as Prime Video or my Synology Nas with DS Video app, but not from Moonlight which is stuck to plain stereo.

I tried different settings, like putting Dolby sounds to OFF or Audio digital format to PCM (from what I read, Nvidia gamestream only supports PCM audio output) but nothing works, still plain stereo in Moonlight.
If I use SPDIF instead of HDMI, then the sound comes from the rear speakers only!

However, if I use Moonlight on my Samsung Tab S6 and connect it to the 5.1 receiver via a HDMI hub, 5.1 sound works fine.

So it has to be the settings from the Connect TV which prevent it.

Any guesses?

Start by reproducing Moonlight on the Connect TV device using the reported HDMI and SPDIF configurations, then compare its audio behavior with the Samsung Tab S6 setup. Confirm the Android TV audio capabilities and Moonlight's negotiated output; the issue is done when surround output works on the reported TV configuration or the limitation is documented.