I tested this with a phone and a cable (Lightning to HDMI adapter) and did see a problem there, although different from what you're describing. The video showed just the first frame on the external display, but actually only played on the main display. It looks like iOS is taking the video to its own player on top of my app, covering up the controls in my app, and ignoring the external display. I'm not sure if I can do anything about that, but I can look into it further if that's your situation. This doesn't happen on a tablet.