On my iPad i can set the system to turn of the screen after 5 minutes. But it will not turn off if I watch a video. This is good.
On my Surface is doesn't work that way. If I set the screen to turn off after 5 minutes it will do so no mather what I do.
I use a Norwegian app called TV 2 Sumo. It's an app from a Norwegian TV-broadcaster where you can watch live and recorded TV. If I set the screen for 15 min the screen will turn of while watching video in this app.
Regarding your problem: Please contact the developer of the app (open charms and tap support) and tell him to implement the blocking of turning-screen off while watching videos. This bug is a common one with new video-apps.