Originally posted by Steam Client Beta - March 19th:

Added more flexible options to the Advanced Host Options for selecting primary display, resolution, refresh rate, etc. while streaming from a Windows computer. This will also allow setting whether HDR is enabled while streaming.

ISSUE: Android phone no longer tells steam it can accept HDR signals. (?)
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Previously my phone would get P010 capture and display HDR content while streaming, however as of the march 19th HDR toggle update it only gets NV12 capture (SDR) despite the host display and client display being HDR. Previously I would stream HDR to my phone. I don't do it often, so it could have broken previously, but I tried all of the toggle switches and it does not work anymore.

I can still stream to my Android TV with HDR. It gets P010 capture and displays HDR content just fine so my host is not the problem.
Toggling HDR always on in the steam settings UI does not enable streamed HDR as expected. Toggling client match does not enable HDR as expected.

ACTUAL RESULT: HDR is not streamed, it's tonemapped down to SDR and looks slightly washed out with limited brightness.
EXPECTED RESULT: is HDR streaming with vivid colors and high brightness.

Steam link app trouble shoot information:
Phone: Steam Link version 1.3.25.302 on Google Pixel 7 Pro - Android 16.
TV: Steam Link version 1.3.25.302 on Mi Box 3 - Android 9

Copied from here: https://steamcommunity.com/groups/SteamClientBeta/discussions/0/803468669855470031/

Hopefully this is the right place to get some traction as it's been a little while. The bug persists on the latest version 1.3.29.308 on Google Pixel 7 Pro - Android 16.