I flew it 3 times.
The first with icons off but using the map to reach the waypoints. At the second WP over the town, the enemy showed up not long after Raven flight arrived. I shot down both, then did a few turns over the city waiting for the last trucks from the airfield. When the last one parked on the side of the road and lost interest in reaching the town over which Raven was still patrolling, I lead the flight south along the planned route. About a half a km before overflying the last waypoint, I received You're leaving the combat area. Thinking that this was it, I exited the mission and saw that the Secondary objective was met, that I had 2 aircraft kills, but not met the Primary objective.
The second time that I flew the mission, I switched icons on to make sure that we didn't miss anything. Again, patrol over the town, kill the bad guys, then fly to the final waypoint. When I received the You're leaving the combat area message again, I circled back, trying to approach the waypoint from the east and west. But received the message each time. So I took the flight to the little encircled area just to the NE of the last waypoint, which I assume is to replicate Bastogne or some place similar to see if we had missed an airfield. Did not see an obvious airfiled to land at. There was one to the NW a short distance, but it was in enemy hands, so avoided it. I looked at the briefing map trying to determine the Primary objective, but failing that, took the flight back to the original airfield and landed. Since I didn't get a Mission Completed message, I assumed that this was also a fail. Now out of ideas, I exited the mission.
On the last attempt to fly the mission, repeated patrolling over the town and shooting down the enemy. BTW, if I could make a small recommendation for the campaign as a whole, wherever there is a flight of 2 EA, I would make it 4, and make them experten. This would increase make things a bit more interesting. Anyway, having done this I had Raven loiter over the city observing the truck behavior for about 15 mins, trying to see if I could determine when they would move on, make a turn, or otherwise do some behavior that would indicate that they are safe. Two of the three trucks at the town finally decided to move down the road, but the other two got stuck in the trees or otherwise off road. Not sure what they were doing, but it looked like they had discovered a wine cellar. Flew to the last waypoint and of course received the leaving the combat are message just before arriving. Throwing up hands at this point.
Did not have a problem in any of the other 13 missions. No big deal. I opened the mission in the editor, trying to determine what changes could be make for me to complete the final mission. But until recently, have been flying mostly DCS, whose mission editor looks nothing like the GB editor. One day I'll take time to learn the editor, and try to determine what I am doing wrong.
Cheers and good luck to all.
Edit: reading @56RAF_Roblex's post again, it looks like it's not the last waypoint that is an issue, but me not getting the Mission Complete statement en route to the last WP. I am following the map's flight path.