If a video is not faded out to complete darkness at the end, no further objects are located behind the last picture objects and the video doesn't have a picture for complete fade-out at the end. The following solutions are possible:
Use a Track Marker for the timeline area to be calculated. See Marking a portion and define the area in a way which places its end to a location behind the last picture object. Under Timeline select Option Only selected portion of timeline for video rendering.
Or place a Position Marker behind the last object. See Markers.