Do you mean that although the ball doesn't across 14-yard line when Diggs is last inbound (the first picture), the ball could be at 14-yard line when any part of his body touches the ground (the last picture)?
If so, the question is not where the ball is when Diggs touches the ground, the culprit is where the ball is when the ball crosses the sideline. By NFL rulebook as below, the spot is determined by the location when the ball crosses the sideline. In this case, it looks to me it's quite short of the first down mark.
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Item 2. Runner Out of Bounds. If the ball is in player possession when that player goes out of bounds,the out-of-bounds spot is the forward point of the ball when the ball crosses the side line, or, if the ball does not cross the sideline, the forward point of the ball at the instant the player is out of bounds
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