Repro: 100% (3/3) in 3den editor
Observed:
Killing the original group leader makes the rest of the group move once another unit has assumed command (see picture 2, note how one group has reached waypoint significantly quicker.)
Once the group leaders(separated from the rest of the group) reaches their waypoints, the rest of the group starts moving into formation (see picture 2).
Issuing a new move order while units are moving into position causes non-leader units to stop on the spot.