I don't think resolution matters when displaying HDR content, since HDR only affects contrast, brightness and colors.
I really can't say what may be happening in my case.
I switched back and forth from the demo to the full game with HDR and I consistently felt that colors looked better in the demo and the full game looked washed out.
I do notice the HDR for example in the contrast between the backlights and the car and stuff like that, but man, I don't know, it does not feel "right" when comparing with how the demo looks.