The example site share by you works well at my end in android and iPhone 6s.
Do you see this in any other specific device?
I cannot see the bug? What do I miss? When you turn your mobile from vertical to landscape your header uses half of the screen?
Or something else, I have not realized?
When switching the video to full screen mode in portrait mode, the video stops. In landscape mode, the full-screen mode works, but also collapses when turning into portrait mode.
Isn`t this youtube-stuff? Don`t know if muse is responsible for that behaviour?
Like Uwe said, this is either a problem with the browser or device, or YouTube.