Also seeing this. Seems like a regression in one of the more recent Steam Deck beta client updates, as the slider appears blue again just by downgrading the Steam client and not SteamOS itself.
This bug was pushed to Stable with Client Update: November 5th.
The Deck Beta client update from today mentions this issue being fixed, seems good here on my OLED Deck so far.
Closing per the last comment.
Nothing extracted yet.
Your system information
Please describe your issue in as much detail as possible:
On the latest (as of writing) Beta branch and in HDR-supported games, the brightness slider in the Quick Access Menu is missing the blue bar to indicate how far to the right the slider is toggled. This issue does not affect non-HDR games.
Steps for reproducing this issue:
Tested in Ratchet & Clank: Rift Apart.
Images attached for both non-HDR and HDR contexts.

