Attacking team kick a grubber into the defending teams in-goal area from just outside the 22.
Defending team gather the ball and choose not to ground it but choose to run it from in-goal.
In the process of running in a large loop the player crosses the dead ball line (under their own momentum no interference from attacking players)
What sanction ?
Law 12.11 – “Apart from at a kick-off or restart kick, if the ball is played or taken into in-goal by an attacking player and is made dead by an opponent, play is restarted with a 22-metre drop-out.”
There is no sanction, but a 22m drop-out.
The ball was put into in-goal by the attacking side and made dead by the defending side (whether intentionally or accidentally) without being brought back into the field of play by the defending side.
Whereabouts on the field of play the ball was kicked from by the attacking side is immaterial. Had the ball been kicked dead by the attacking side (without touching an opponent) then the defending side would have had the option of 22-metre dropout or a scrum at the point from where the ball was kicked (Law 21.12).
An obligation on the defender to “ground the ball without delay” in order to benefit only applies to Kick-off and restart kicks (Law 12.9).
Response written January 3rd, 2019.