Google has made a change to the resolution options in the YouTube mobile app for HDR videos. The resolution now maxes out at 1080p, regardless of the display of your phone.Resolution options on the Galaxy Note8
Previously, the resolution options went all the way up to 4K, provided you had a Sony Xperia XZ Premium. For most other flagship it was going up to 1440p.
The limit is likely being put because of how bad HDR video performance is in the YouTube app. Even on flagship devices, including Google's own Pixel 2, playing HDR videos over 720p caused massive frame drops and a generally unusable experience.
The YouTube app is alone in this, as other apps with HDR support, such as Netflix and Amazon Prime Video, have no issues with HDR videos up to 4K resolution.
Google's solution is rather strange, where instead of improving playback performance the higher resolution options are now simply obscured from the user.