Download the mission file, install the mission and load it up. Enter Commander mode using Ctrl+Space. Make a waypoint for one of the infantry groups (not the motorized infantry group or the chopper), right click the waypoint before the group reaches it and select Type->GET IN. The first infantry group will probably board the empty truck. If the first group entered the empty vehicle, it means you made the waypoint within 50m of the empty truck. In that case select the second infantry group and try to make them board something. Now you will notice the bug as described (if you haven't already). You can further test heli-extractions and vehicles grouped to infantry in the mission, if you want to.