Unity does not support HDR output with exclusive fullscreen in DirectX 11. HDR output requires flip model swapchain, but Unity uses bitblt model in exclusive fullscreen mode for performance reason. You can find relevant information here:
- https://docs.unity3d.com/6000.3/Documentation/ScriptReference/PlayerSettings-useFlipModelSwapchain.html
- https://docs.unity3d.com/6000.3/Documentation/Manual/urp/post-processing/hdr-output.html