I can see the dilemma of setting the time at which video thumbnails are generated. Set it too early, and movie thumbnails are still black because the movie hasn't started properly. Set it too late and short video files won't have any thumbnails.
Perhaps there is a way to set it for video duration divided by 3, or percentage of video's duration. It's just an idea no idea if ffmpeg supports that kind of input instead of an exaclt amount in seconds.