Try it with different monitors you have. The current nVidia Linux drivers only has BGR output for 10bpp, which works on TVs and OLEDs but not most LCDs monitors.
My monitors (InnoCN 27M2V and Cooler Master GP27U) require RGB input, which means it's limited to 8bpp even with HDR enabled on Wayland. There's another commentator below who uses a Dell monitor and manages to get BGR input working and full HDR in nVidia/Linux.
I don't have a Dell U2725QE, but on InnoCN 27M2V and Cooler Master GP27U there's no ABGR2101010 support. These monitors would only work with ARGB2101010 or XRGB2101010 which nVidia drivers do not provide.
They're also selling $3000 nVidia AI workstations that exclusively uses Linux. But what if you want to watch an HDR video on it? No. What if you want to use Google Meet on Chrome/Wayland? It's broken.
You can enable, yes. But (assuming you're on an LCD display and not an OLED), you're likely still on XRGB8888 - i.e. 8-bit per channel. Check `drm_info`.
Do it once on "HDR" on Linux, and then on Windows. The "HDR" in nVidia/Linux is fake.
The brightness you see on Plasma or Mutter is indeed related to the HDR support in the driver. But - it's not really useful for the most common HDR tasks at the moment.
HDR still doesn't really work on Linux w/ nVidia GPUs.
1. 10bpp color depth is not supported on RGB monitors, which are the majority of LCD displays on the market. Concretely, ARGB2101010 and XRGB2101010 modes are not supported by current nVidia Linux drivers - the drivers only offer ABGR2101010 and XBGR2101010 (See: https://github.com/NVIDIA/open-gpu-kernel-modules/blob/main/...).
2. Common browsers like Chrome and Firefox has no real support for HDR video playback on nVidia Linux drivers. The "HDR" option appears on YouTube, but no HDR color can be displayed with an nVidia GPU.
Also, video backgrounds in Google Meet on Chrome are broken with nVidia GPUs and Wayland. Ironically it works on Firefox. This has been broken for a few years and no fix is in sight.
The "HDR" toggle you get on Plasma or Mutter is hiding a ton of problems behind the scenes. If you only have 8bpp, even if you can find an app that somehow displays HDR colors on nVidia/Wayland - you'll see artifacts on color gradients.
Imagine someone outside of the tech community thinking along this line...
"Making high performance CPUs that are also highly power efficient should make a ton of money. Why isn't everyone doing it?"
Well, turns out that isn't exactly something that a small group of engineers can whip up in a garage anymore. Same goes for highly efficient market making systems.
My monitors (InnoCN 27M2V and Cooler Master GP27U) require RGB input, which means it's limited to 8bpp even with HDR enabled on Wayland. There's another commentator below who uses a Dell monitor and manages to get BGR input working and full HDR in nVidia/Linux.