YouTube appears to be working to bring 4K High Dynamic Range (HDR) playback back to its platform, following a cap placed on the content earlier this month which set the maximum resolution at 1080p. That's according to a response from at least one Googler, which was posted to the official help forums in response to an inquiry about the cap for one user's Xbox One X console. It is important to note that the response in question was also only addressed toward playback on one specific type of game console, with regard to the YouTube application for that console. While it stands to reason that the company would not want to ignore the mobile side of its business, with many of the newest Android devices supporting HDR playback, this response does not guarantee a fix for mobile users.
As of this writing, the company has not provided any reason for why support for HDR content played through the mobile app or elsewhere at higher resolutions was suddenly dropped. In fact, prior to this response, and possibly other responses like it, there had not even been an official confirmation of any kind to indicate that a cap had been deliberately put in place. Bearing that in mind, the implementation of the cap itself seems to have occurred only a couple of months after content started to be offered at higher resolutions for users with compatible hardware. So it is possible, and even arguably likely, that offering the resolution resulted in issues of some kind and that Google responded by limited playback until a fix could be found. That would make some sense since the company wouldn't want to be responsible for distributing content with laggy playback or other issues.
However, no new details were provided about what, if anything, the problem seems to be. Instead, the Googler in question only stated that 4K content is not currently being offered and is limited to 1080p, but that the company is working on it. Unfortunately, given that there has been no date provided for a fix either, it could be a while before resolutions above 1080p are available again on Android.