Enabling HDR rendering messes with much deeper stuff than just the graphics, so without full system info it's unlikely that anyone can unriddle this. A simple "bad" HDMI cable or something can have weird effects. I could even imagine a DRM-related issue because Windows thinks you are using some media componets for 4k HDR playback. Of course it can always be a bug, but even that would require more detailed info.
Because HDR being enabled and GPU acceleration work flawlessly in Lightroom - it seems unlikely this is a hardware issue. I would question the entire community if anyone is able to use GPU acceleration with HDR enabled, before assuming my hardware/windows setup has anything to do with it. I have a feeling its either a bug or something broken with NVIDIA cards.
I take it Adobe doesn't respond to most bug reports??