The issue is By Design. It's mentioned that the built-in Camera target RT format is R8B8G8A8_SRGB and the texture user gets is R8B8G8A8_UNorm.CopyTexture should just do a memcopy without altering the data and _UNorm vs _SRGB affects how the colors are sampled so it's simply an issue of different formats which is by design. There is a file called XRHelper.cs, where the developer have done some changes which will return a default format rather than UNORM:

public static RenderTextureDescriptor GetRenderTextureDescriptor(Camera camera)
{
#if _XR_ENABLED
if (IsRunning)
{
return XRSettings.eyeTextureDesc;
}
else
#endif
{
// Use a different constructor here passing an explicit GraphicsFormat that's based on DefaultFormat and the current colorspace. This should match the color targets the user wants to copy from.
return new RenderTextureDescriptor(camera.pixelWidth, camera.pixelHeight, SystemInfo.GetGraphicsFormat(DefaultFormat.LDR), 0);
}
}