Update: Looks like this issue is isolated to Android and only a handful of specific cases. We're continuing to monitor reports to get this fixed!https://twitter.com/TeamYouTube/status/987041038844874752 …
Looks like you've got a lot on your plate but nonetheless I'd like you to make the YouTube app so that portrait videos can be viewed full screen on 18.5:9 displays and that natively wide videos take up as much room as they can when in full screen