Steam Machine breaks HDMI-CEC for all other connected devices

[espiox]

Your system information

• Steam client version: 1789086785
• SteamOS version: 3.8.27
• 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:

When I turn the Steam Machine on, not only do the CEC features not work, it stops CEC working for every other device connected to the TV, including wake and volume control features. My TV is a Samsung UE55MU7000, and this is my HDMI setup:

• HDMI1: Steam Machine
• HDMI2: PS5
• HDMI3: Switch 2 Dock
• HDMI4 (ARC): Yamaha YAS-107 soundbar, with Apple TV 4K (3rd-gen) connected via HDMI passthrough

I've tried disabling every CEC-related setting under Power and Audio in Steam's settings, but that doesn't seem to stop the CEC signal completely, and it still breaks everything.

Steps for reproducing this issue:

1. Normally, the Apple TV/PS5/Switch 2 all turn on the TV when woken up, and the TV/Apple TV are able to control soundbar audio.
2. As soon as the Steam Machine is turned on, neither my TV nor devices can interact with the soundbar any more, and CEC wake commands no longer work for any device including the Steam Machine.
3. Fully power cycling everything solves the issue - until the Steam Machine is powered back on.

In 8 years with this TV, I've never had this issue with a CEC device before.