LG C2 audio transcoding always plays as stereo

[aedalzotto]

Describe The Bug
Unsupported audio tracks correctly trigger audio transcode.
Although multichannel audio is transcoded correctly to multichannel AAC (or AC3, when selected), the TV passes through stereo PCM through eARC.
This behavior occurs both using the TV browser and the jellyfin-webos application.
This behavior occurs regardless of the maximum channels option being set to 5.1/7.1

Steps To Reproduce

1. Play an unsupported multichannel audio track (tested with DTS[-HD MA] and FLAC 5.1) or lower the playback quality to force a transcode due to bitrate
2. Verify using Playback Info that the client receives multichannel AAC (or multichannel AC3, when opted in client playback options)
3. Check the eARC receiver to state that it is, in fact, receiving stereo PCM

Expected Behavior

• If transcoding to multichannel AAC, the TV should play multichannel PCM through eARC. I have verified that HE-AAC and AAC-LC correctly play as multichannel PCM when not transcoding.
• If transcoding to multichannel AC3, the TV should pass through the codec through eARC. I have verified that AC3 correctly passes through eARC when not transcoding.

Logs
jellyfin-server log with DTS-HD MA to AAC 6-channel

[INF] /usr/lib/jellyfin-ffmpeg/ffmpeg -analyzeduration 200M -probesize 1G -ss 00:10:12.875 -readrate 10 -fflags +genpts -f matroska  -i file:"_.mkv" -map_metadata -1 -map_chapters -1 -threads 0 -map 0:0 -map 0:1 -map -0:s -codec:v:0 copy -bsf:v h264_mp4toannexb -start_at_zero -codec:a:0 aac -ac 6 -ab 640000 -copyts -avoid_negative_ts disabled -max_muxing_queue_size 2048 -f hls -max_delay 5000000 -hls_time 6 -hls_segment_type mpegts -start_number 102 -hls_segment_filename "_%d.ts" -hls_playlist_type vod -hls_list_size 0 -y "_.m3u8"

System (please complete the following information):

• Platform: LG OLED C2 v23.20.56 (recent webOS 24 update). Both with Jellyfin v1.2.2 and using the TV browser.
• Jellyfin Version: 10.10.5

Additional Context
Additional bug: jellyfin-webos supports Opus playback, but when opting to transcode unsupported tracks to Opus, it requests the server for AAC.

Edit: This seems webOS specific, but it needs further testing with different versions since my TV was updated to webOS24 this January. I've tested with a Windows laptop connected directly to the eARC receiver and transcoded AAC shows as multichannel PCM, while transcoded AC3 gets passed through directly (correct behavior).

[aedalzotto]

I've realized that the problem is occurring not only with transcoding.
I've tried to play a mkv Dolby Vision media, which is unsupported and triggers a remux.
Jellyfin on TV receives the remuxed stream with both video and audio as direct.
The audio track is EAC3, which is normally passed through eARC, but when remuxing, although still direct playing, gets sent as stereo PCM.

[buletti]

For the record: I have reported a similar Stereo-Only Issue for Dolby Vision streams with jellyfin/jellyfin-webos#275.