If a group owns a vehicle, they can no longer be given waypoints to get into another vehicle by synchronizing a "Get in" with a "Load" waypoint of another group.
See the attached mission. Simply preview it. The only difference between the two groups is that one will first get into a car, then out, causing them to own it. They now won't get into the helicopter like the other group.
I first thought it was an AI bug, but it seems to be a bug with the "Get in" waypoint. If a group owns a vehicle, it will not snap onto the helicopter, but just behave like it was placed spatially without synchronization. {F24897}