FDefaultGameMoviePlayer always render a black image?

FLoadingScreenAttributes LoadingScreenAttr;
LoadingScreenAttr.MinimumLoadingScreenDisplayTime = -1.0f;
LoadingScreenAttr.bAutoCompleteWhenLoadingCompletes = false; 
LoadingScreenAttr.bAllowEngineTick = false; 
LoadingScreenAttr.bMoviesAreSkippable = false;
LoadingScreenAttr.bWaitForManualStop = false; 
LoadingScreenAttr.MaxPlayMovieCount = 1;
LoadingScreenAttr.PlaybackType = EMoviePlaybackType::MT_Normal;

Hi everyone ! I encounter a problem.
Before OpenLevel,I use code above to play loading movie,but every time the movie finish, I can see a black image on the screen, then one (maybe two?)frame later I am able to enter the level,
looks like FDefaultGameMoviePlayer render a empty texture more frame.
Why this happens? Im looking forward to your answers, thanks very much!