XAML MediaPlayerElement control/class is exhibiting a ridiculous bug on Xbox One: once you are going fullscreen the control is not showing the video correctly. A texture is assumed to be rendered in full and it is cropped instead (left top quarter of actual video or alike).
I saw this before but at that time I had other reasons to not use built-in fullscreen mode. This time the reason is more straightforward – it simply does not work as expected.