colorspace-hint does not signal the display to use HLG transfer function

Important Information

• mpv version: sourceforge latest as of Sep 21st 2023
• Windows Version: WIndows 11 Build 22623
• GPU: Nvidia GTX 1650 Super
• Source of the mpv binary: https://sourceforge.net/projects/mpv-player-windows/files/latest/download
• Screenshot or video: https://slow.pics/c/FApJBfXy - slightly exaggerated visualisation of ideal behavior vs actual behavior

Reproduction steps

When you run an HDR video with the following parameters, an HDR compatible display is signaled to switch to its HDR mode with PQ transfer, and the video is displayed correctly when in fullscreen. It does not trigger the Windows HDR toggle (making everything outside of mpv look off with the PQ trc, but no matter).
--vo=gpu-next --target-colorspace-hint=yes --gpu-api=vulkan
However, when playing an HLG video on a supported display, it would be optimal to signal the display to run with HLG trc, but it still runs in PQ. I'm using an LG C1, with support for HDR10, HLG and DV. By using --target-trc=hlg and then manually going into the display's HDMI override settings to switch to HLG TRC, I can get the intended look of the video, which is what you'll find the in comparison above.

Is this a bug? Or is HLG signaling avoided in favor of PQ for compatibility reasons?

Log file

output.txt

Sample files

This is the sample used in the comparison https://files.fm/u/nu8tucfq7
But any HLG file will have the same results.

Start by reproducing the issue with --vo=gpu-next, --target-colorspace-hint=yes, and --gpu-api=vulkan using an HLG sample, then inspect output.txt and the behavior described in the comparison. Trace how --target-trc=hlg and the display signaling are handled; done means HLG content produces the intended display transfer behavior, or the compatibility rationale is documented.