I have experienced this bug repeatedly in several scenarios. The 9-man squad is not split and has never been split, yet half the squad moves off leaving the other half behind with the single icon remaining halfway between the 2 "teams". The only way I have found to get around this problem is, as others have suggested, to manually split the squad with a Split order and then move the 2 split teams back together.
I have also seen this problem happen with a 3-man US MG team. 2 men moved across and then along a road as ordered leaving the 3rd on the original side of the road and never moving. The icon was midway between the now 2 groups. However, I did manage to get them to re-group automatically when I set the next waypoint, although I did set this back on the original side of the road where the single MG guy was. It was as if the original guy not moving was stuck and could not cross the road at that point for some reason I do not know. He eventually did move parallel to the road and joined back up when I gave the order for his team mates to cross back to the original side of the road.
Rob