Jellyfin MPV-Shim flaypack doesn't support passthrough multichannel audio over HDMI

[chp-1960]

Describe the bug
Jellyfin MPV-Shim flatpack doesn't support passthrough multichannel audio over HDMI with integrated MPV.
The tests were made with the following materials:

Sound cards tested :

• HDMI to SPDIF splitter or/and
• External USB sound card with analog and SPDIF out/in
• AC3 and DTS test videos.
• Amplifier : Onkyo
• Tested with ALSA, Pulse and Pipewire

Nota : Regular mpv and vlc are working as expected (SPDIF passthrough works).

In Jellyfin MPV-Shim, both sound cards have the same behavior : No SPDIF passthrough. If passthrough is forced, in home/user/.var/app/com.github.iwalton3.jellyfin-mpv-shim/config/jellyfin-mpv-shim/mpv.conf the embedded MPV never open.

However, it work in stereo (prologic at best) if no sound card is selected in the mpv.conf or if it's empty.

To Reproduce
I tried to modify the file mpv.conf located in : home/user/.var/app/com.github.iwalton3.jellyfin-mpv-shim/config/jellyfin-mpv-shim/ to use the exact same parameters as the regular mpv but without success. At best I can have stereo (prologic) sound.
If I set the same sound card as the "regular" mpv, the "embeded flatpack mpv" don't open at all.

Expected behavior
All AC3, DTS, EAC3, ... multichannel audio should go to the Amp/Receiver in "spdif passthrough".

Screenshots
not applicable.
Here is my common settings for both "regular" mpv and for the "embedded flatpack" MPV :

mpv.conf :
audio-device='alsa/hdmi:CARD=Generic,DEV=0'
audio-channels=5.1,stereo # audio configuration with fall back option
af=surround:chl_out=5.1 # audio filter for mixing to surround (5.1)
audio-spdif=ac3,dts,eac3 # audio passthrough on HDMI or SPDIF

Desktop (please complete the following information):

• OS: Kubuntu 22.04 LTS
• Browser : Firefox

Error Messages
Where and which logs show the mpv results and capabilities?