Why did 3rd umpire Chris Gaffaney call what would have been the last ball of the Bangladesh-Zimbabwe game a no-ball?

Bangladesh had already celebrated a narrow win and had walked off the field.
On replays checking for the stumping of Blessing Muzarabani off Mosaddek Hossain, wicketkeeper Nurul Hasan was seen to have collected the balll before it had passed the stumps.
Here is what the law states:
27.3 Position of wicket-keeper
27.3.1 The wicket-keeper shall remain wholly behind the wicket at the striker’s end from the moment the ball comes into play until a ball delivered the bowler touches the bat or person of the striker or passes the wicket at the striker’s end or the striker attempts a run.
27.3.2 In the event of the wicket-keeper contravening this Law, the striker’s end umpire shall call and signal No ball as soon as applicable after the delivery of the ball.