It looks like, this week, Youtube updated the video embedding. All videos now play in a custom interface instead of the traditional one.
Unfortunately, in iOS at least, the new interface doesn’t allow to play videos in full screen. Even if you use Safari, it says that the browser doesn’t support the feature - my opinion is that clearly, Youtube is forcing people to use their apps.
It would be great if we could have a way to use oldest versions of the embed code, custom embed codes or even a workaround to enable the fullscreen play.
Play a video in a mobile application without the possibility to watch in full screen doesn’t make sense at all. It’s a bit unfair on how Youtube runs the game. ):