I've played the clip back on YouTube at 1/4 speed and you can hear the whistle starting to go about 1 meter our before the ball crossed the line. But that doesn't cover up that Mason had no legal right to blow again according to the rules of the game.
Whether has had a ‘right’ to blow again isn’t the point though is it ? He did blow again, so play immediately stops - so if the ball wasn’t over the line, it wasn’t a goal. Refs often blow when they shouldn’t have (fouls that aren’t fouls etc). Play stops.
Why he blew again is another question..............that hasn’t been answered as far as I know.