It is clear they needed to give Mason a warning and told him to back up and not cross the plane. They missed it. I will say a game of that intensity with so many make/break calls, the crew for the most part did outstanding. There were literally thousands of judgment calls made and officials are destined to miss a few. They aren't robots. It just sucks that this one happened during a critical point in the game.
Like the players, I would bet they were also rather fatigued. Personally, I am puzzled most on the first of the two non-calls on interference/goaltending.