By default, OSMF 2 that is new in 4.11 bundle use stageVideo when goes to fullscreen. and the result is that by default, a flex projects using a VideoPlayer component, will get no image when in fullscreen, because of component Rect background that will be drawn above video image, in stageVideo mode.
add OSMFSettings.enableStageVideo = false; call before the CreationComplete event dispatch of the main Application.
So that we are sure that stageVideo is not used, as it was for previous versions.
But theres still a way to get stageVideo enabled in a flex projects.