1.2.0.98 No Mods
Also happened in 0.92
AI squad members unable to re-enter a vehicle.
Playing Combat Ops single player PvE, created a squad and got them to board a truck. Drove to objective and got them to get out. Engaged in a couple of firefights, ordered them to get back in the vehicle, they assembled and looked like they would get in but they would not get in.
Tried spawning another truck, same problem.
Pretty sure this behaviour persisted after I died and re-spawned.