Per-game manually set GPU clock is not consistently applied on game launch in Game Mode on Steam Deck (SteamOS 3.8)

Reproduce the launch sequence on a Steam Deck running SteamOS 3.8.14 with a per-game manual GPU clock, using Performance Overlay Level 3 to compare launches. Investigate the Game Mode performance-setting application path and compare it with the in-game toggle; done means the configured clock is applied consistently on every launch without manual re-toggling.

Your system information

• Steam client version:1782861641
• SteamOS version: SteamOS 3.8.14 Build: 20260703.1
• Opted into Steam client beta?: No
• Opted into SteamOS beta?: No
• Have you checked for updates in Settings > System?: Yes

Please describe your issue in as much detail as possible:

Summary

Since updating my Steam Deck OLED 512 GB to SteamOS 3.8, the per-game TDP setting appears to apply correctly on every game launch. However, the per-game manual GPU clock setting does not consistently apply when starting a game in Game Mode.
On SteamOS 3.7 and earlier, the issue primarily affected the per-game TDP setting, which often failed to apply correctly. After updating to SteamOS 3.8, that issue appears to be resolved, but a similar inconsistency now affects the manually configured GPU clock.

Example

Using Forza Horizon 4 with Proton Experimental, the per-game GPU clock is set to 1300 MHz.

• Occasionally, the game launches and the GPU clock is correctly locked at 1300 MHz.
• Most launches, however, the GPU clock remains variable, even though the Performance menu still shows the GPU clock slider set to Manual with 1300 MHz selected.

Restarting or fully shutting down and rebooting the Steam Deck does not improve the consistency.

As a workaround, disabling and immediately re-enabling the manual GPU clock toggle while the game is already running causes the GPU clock to immediately lock to the configured value and remain there for the rest of the session.

Steps for reproducing this issue:

1. Start the Steam Deck.
2. Launch a game that has per-game Performance settings configured with a manually set GPU clock.
3. Enable at least Performance Overlay Level 3 so the GPU clock is visible.
4. Observe whether the GPU clock is locked to the configured manual value after the game launches.

Expected Result

The manually configured per-game GPU clock should be applied every time the game is launched, just like the per-game TDP setting.

Actual Result

The manual GPU clock is applied only intermittently. On most launches, the GPU clock remains variable despite the Performance menu indicating that Manual GPU Clock is enabled. Toggling the manual GPU clock off and back on while in-game immediately applies the configured clock correctly.