I first observed this issue in either 1.36 or 1.38,and it still occurs in 1.40,but I haven't played before version 1.36 and do not know when it first occurred.
It does not seem to matter whether the group leader is infantry or vehicle crew,although infantry may be more likely to have some infantry but not all re board the vehicle.
Still a problem in 1.44 and what confuses me is there isn't this problem in old A2OA,there the infantry seem to re board the vehicle the minute the enemy die or sometimes sooner to continue on,yet this is a problem in A3,where when a motorized/mechanized group encounters the enemy their left running after their vehicles long after the enemy is gone until they reach a waypoint that's still far away 99% of the time,and only once reaching the waypoint might re board the vehicle.