This isn’t specific to Nothing, but other brands like Samsung and Pixel also have the same problems according to Reddit. That’s why I am not sending a feedback report yet. I’m making this post because I’ve seen many HDR related posts here that don’t fully explain what’s wrong.
At high phone brightness, HDR content is less bright
Pardon the bad quality. I tested Apple TV+ HDR:

The left one is much better because in some dark scenes it becomes hard to see anything due to the dimming, even at high brightness
Whone brightness is turned all the way down, HDR is still middle to bright
(I’m taking some more pics for this part, will edit in a bit)
Temporary solution: Disable HDR playback
I followed this great reddit post using some adb commands: https://www.reddit.com/r/Android/comments/16w46l5/tip_how_to_turn_off_hdr_video_playback_on/ According to OP, it should still play HDR for local files but it’s good enough