That’s the normal behaviour actually. Only a poster image (static) will be displayed. That’s because videos on smartphones or general smartphone operating systems are very problematic. Some of the problems are:

• They take over the video, modify the z-layering structure, for example z-index. This alone generates a ton of problems;
• OS’s add own controls which are built-in the operating system and are not customisable;
• Can’t autoplay a video (bandwidth consumption protection)
• Can’t override any system option.

What will do work is videos that are inside a player, for example a video that’s launched in a modal window, or a video embeded into the article.

So basically a video background should do what it supposed to, to display a video in the background. Unfortunately the problems of having them enabled on mobiles are too big so this is turned off by default.