Back in September, Google added the ability to play HDR content to the YouTube Android app. It's still only available to a handful of devices, but even on those, many noticed performance issues on higher resolutions. Especially at 1440p, videos often stuttered with dropped frames.
Now it seems like the option to play 1440p (or higher) HDR videos has been completely removed from the mobile apps. Perhaps Google couldn't find a way to fix video playback issues, or the current batch of flagships just aren't up to the task. 1080p is now the maximum resolution for HDR in YouTube.