UIViewController's view has weird 20px

Hello Guys I am facing a wierd behaviour in my app the case is like this :

From view controller(A) present another view controller (B) which has a UIWebView that will open an URL, but sometime the URL is Video link, Sot he video starts playing in full screen and when finish and tap on Done (Dismiss Video player) and then dismiss the Controller (B) a weird 20px appeared at the bottom of view controller(A) and the whole app will lose the 20px in all screens.

Any one has idea for what is happening, I am certain sure that it caused by playing video through UIWebView

Update 1 : I just figured out that this could be related to the iOS itself when playing videos whether from video file or from video url (since both will be played using the same player). My experiment was on Facebook app when I post the same links used in my app inside Facebook app then try to play the videos from the links and the behaviour showed !

I wish if could someone help to confirm if playing videos causing problems on the UI.