Here's an example -
http://goallineblitz.com/game/replay.pl?game_id=2429463&pbp_id=7130192FS is set to cover zone immediately - Flat Weak Third
He never even makes a move toward his zone. See this all the time.
My guess is that it's coded so that the defender has to pass a vision check to determine if the play is a pass play or a run play - if pass then they go to zone, if run then they make a beeline for the ball carrier. I can understand that Bort does not want to mess with those mechanics and open up a potential world of other problems, but he should fix it so that the defender goes to his assigned zone when 'cover zone immediately' is called for regardless of the play type, then defends the pass or the run.
There is no qualification that says 'cover zone immediately unless...'. It should do what it says it does.