Steamdeck "Gaming Mode" sets incorrect display mode when connected to external display via AV receiver

[andersnylander]

Your system information

• Steam client version: Steam package versions: 1657232451
• SteamOS version: 3.3 Preview
• Opted into Steam client beta?: Yes
• Opted into SteamOS beta?: Yes
• Have you checked for updates in Settings > System?: Yes

Please describe your issue in as much detail as possible:

My Setup: Steamdeck UGreen 4K 60Hz USB-C Hub Sony Multi-Channel AV Receiver STR-DH590 LG OLED55CX6LA The deck connects to the USB-C Hub via USB-C cable, and there are HDMI 2.0 cables connecting the USB-C hub to the AV receiver, and also connecting the receiver to the TV (via the HDMI port marked as ARC on the TV).

Expected the Steamdeck "gaming mode" client to detect and output 3840x2160 @60Hz. Actual resolution output was 640x480 @60Hz. In desktop mode, the output mode is correctly set to 3840x2160 @60Hz.

Also, when connecting the USB-C hub to the TV directly, the mode is correctly set to 3840x2160 @60Hz, both for desktop and gaming mode. However when doing so, due to limitations in the TV, multichannel audio (>2 channel PCM) does not get passed through. Though this may be a hint for how the steam "gaming mode" display stack is behaving differently from the desktop mode.

Steps for reproducing this issue:

Recreate the setup above as closely as possible.