It is not a foul to have a hand on an opponent's face mask.
It is a face mask foul if a player grasps an opponent’s face mask, any edge of a helmet opening, chin strap, or a tooth and mouth protector attached to the face mask and twist, turn or pull the face mask, helmet opening, chin strap or a tooth and mouth protector attached to the face mask; or incidentally grasp an opponent’s face mask, helmet opening, chin strap or a tooth and mouth protector attached to the face mask.
This picture captures a split second in time and freezes it. You don't see the whole act. It may have happened so fast that the fingers release before the covering official(s) sees this..... or the view of covering official(s) was obstructed by other players...// or maybe they just missed it, etc.......
It's why still pics are not good examples to use to show missed or correct calls by officials.