Experiencing the same issue. Dolby plays fine, but any non-Dolby 4k content does not trigger HDR. All other apps work correctly.
Someone pointed out to me that it's because the content i was watching was actually in 4k Ultra HD and not HDR. I think that was the issue. I haven't been able to find any HDR conent that is not dolby vision in netflix yet so i can't be for sure until i do find some.
That makes sense. Thanks for the info!