Desktop Mode games fail to render (blank screen until alt-tab) on RX 9070 XT — SteamOS 3.8.25 Beta

Summary:
On SteamOS 3.8.25 (Beta/Preview channel), games launched via Steam in Desktop Mode load correctly in the background but render a blank/black window. Switching focus away and back (alt-tab) forces a repaint and the game displays correctly afterward. Affects all games tested, not isolated to one title. Games launched via Gaming Mode are unaffected.

System info:

OS: SteamOS 3.8.25 (Beta), BUILD_ID 20260807.2
GPU: AMD Radeon RX 9070 XT (RDNA4)
CPU: AMD Ryzen (with Raphael integrated graphics, confirmed not the cause — disabling iGPU did not change behavior)
Monitor: 3440x1440 ultrawide via HDMI

Relevant dmesg output at boot:

amdgpu 0000:7c:00.0: [drm] Cannot find any crtc or sizes
amdgpu 0000:03:00.0: [drm] REG_WAIT timeout 1us * 150000 tries - optc401_disable_crtc line:235

Relevant journalctl output during gamescope session stop/restart:

gamescope-session: (EE) failed to read Wayland events: Broken pipe
gamescope-session: [gamescope] [Error] drm: drmModeRmFB failed: Bad file descriptor

Troubleshooting already performed:

Confirmed monitor cable connected to GPU output, not motherboard
Disabled integrated graphics in BIOS — no change
Confirmed steamos-readonly was enabled (not a filesystem write issue)
Ran steamos-update check — no update available on Beta channel at time of testing
Found a closely matching report on Arch Linux forums with the identical optc401_disable_crtc timeout on a different distro, suggesting an upstream amdgpu/DCN kernel issue rather than SteamOS-specific

Resolution found:
Switching the SteamOS System Update Channel from Beta/Preview back to Stable (3.8.14) resolved the issue completely — games now render correctly in Desktop Mode without needing gamescope launch option workarounds. This strongly suggests a regression introduced in the 3.8.25 Beta build specific to RDNA4 GPUs.

Workaround (for Beta channel users who can't roll back):

gamescope -O <output_name> -W -H -w -h -b -- %command%

Start by reproducing Desktop Mode rendering on SteamOS 3.8.25 Beta with the listed RX 9070 XT setup, then review the provided dmesg and journalctl output. Compare behavior with Stable 3.8.14 and the gamescope workaround. Done means identifying the regression or its upstream owner and restoring normal rendering without alt-tab or launch-option workarounds.