Problem dropping specific packages on specific points using or-tools

Question

I am currently working on a project that needs to optimize the travel path of a certain number of vehicles. Now, at first, I can consider all my vehicles (only one, at first) starting from the same spot. However, I need to make the following travels:

data['pickups_deliveries'] = [
   [1, 2],
   [3, 4],
   [2, 5],
]

Note that node 2 is both a pickup point and a delivery point.

While doing this travels, I need to:

• Pick something on 1 --> to drop it on 2. (the object I pick in 1, must go to 2. Can't go anywhere else. For example, it may be a machine that only works only on a factory located on 2)
• Pick something on 3 --> drop it on 4. Same here...
• Pick something on 2(6) --> drop it on 5. Same here. What I pick in 2/6 is only useful on 5.

data['pickups_deliveries'] = [
  [1, 2], #1,2 
  [3, 4], #3,4
  [6, 5], #5,6
  ]

In order to make this happen, I understand that node 2 has to be duplicated. Thus, I made a node 6 and add an extra column and an extra row to the distance matrix (element [6,2]=[2,6]=0, as it is the same node.)

The base is at node 0. Code below:

"""Capacity Vehicles Routing Problem (CVRP)."""

def create_data_model():
    """Stores the data for the problem."""
    data = {}
    data['distance_matrix'] = [
       [0   , 2200, 4600, 7000, 2800, 4600, 4600],
       [2200, 0   , 5700, 6700, 5000, 3800, 5700],
       [4600, 5700, 0   , 3500, 5500, 3400, 0   ],
       [7000, 6700, 3500, 0   , 8500, 1800, 3500],
       [2800, 5000, 5500, 8500, 0   , 6300, 5500],
       [4600, 3800, 3400, 1800, 6300,    0, 3400],
       [4600, 5700,    0, 3500, 5500, 3400,    0],
    ]
    data['pickups_deliveries'] = [
        [1, 2], #1,2 
        [3, 4], #3,4
        [6, 5], #5,6
    ]
    data['demands'] = [0,  # departs empty
        1,  # load in 1
        -1, # unload in 2
        1,  # load in 3
        -1, # unload 4
        1,  # load in 6 (duplicate of 2)
        -1] # unload in 5
    data['vehicle_capacities'] = [1]
    data['num_vehicles'] = 1
    data['depot'] = 0
    return data


def print_solution(data, manager, routing, solution):
    """Prints solution on console."""
    print(f'Objective: {solution.ObjectiveValue()}')
    total_distance = 0
    total_load = 0
    for vehicle_id in range(data['num_vehicles']):
        index = routing.Start(vehicle_id)
        plan_output = 'Route for vehicle {}:\n'.format(vehicle_id)
        route_distance = 0
        route_load = 0
        while not routing.IsEnd(index):
            node_index = manager.IndexToNode(index)
            route_load += data['demands'][node_index]
            plan_output += ' {0} Load({1}) -> '.format(node_index, route_load)
            previous_index = index
            index = solution.Value(routing.NextVar(index))
            route_distance += routing.GetArcCostForVehicle(
                previous_index, index, vehicle_id)
        plan_output += ' {0} Load({1})\n'.format(manager.IndexToNode(index),
                                                 route_load)
        plan_output += 'Distance of the route: {}m\n'.format(route_distance)
        plan_output += 'Load of the route: {}\n'.format(route_load)
        print(plan_output)
        total_distance += route_distance
        total_load += route_load
    print('Total distance of all routes: {}m'.format(total_distance))
    print('Total load of all routes: {}'.format(total_load))


def main():
    """Solve the CVRP problem."""
    # Instantiate the data problem.
    data = create_data_model()

    # Create the routing index manager.
    manager = pywrapcp.RoutingIndexManager(len(data['distance_matrix']),
                                           data['num_vehicles'], data['depot'])

    # Create Routing Model.
    routing = pywrapcp.RoutingModel(manager)


    # Create and register a transit callback.
    def distance_callback(from_index, to_index):
        """Returns the distance between the two nodes."""
        # Convert from routing variable Index to distance matrix NodeIndex.
        from_node = manager.IndexToNode(from_index)
        to_node = manager.IndexToNode(to_index)
        return data['distance_matrix'][from_node][to_node]

    transit_callback_index = routing.RegisterTransitCallback(distance_callback)

    # Define cost of each arc.
    routing.SetArcCostEvaluatorOfAllVehicles(transit_callback_index)


    # Add Capacity constraint.
    def demand_callback(from_index):
        """Returns the demand of the node."""
        # Convert from routing variable Index to demands NodeIndex.
        from_node = manager.IndexToNode(from_index)
        return data['demands'][from_node]

    demand_callback_index = routing.RegisterUnaryTransitCallback(demand_callback)
    
    routing.AddDimensionWithVehicleCapacity(
        demand_callback_index,
        0,  # null capacity slack
        data['vehicle_capacities'],  # vehicle maximum capacities
        True,  # start cumul to zero
        'Capacity')

    # Setting first solution heuristic.
    search_parameters = pywrapcp.DefaultRoutingSearchParameters()
    search_parameters.first_solution_strategy = (routing_enums_pb2.FirstSolutionStrategy.PATH_CHEAPEST_ARC)
    search_parameters.local_search_metaheuristic = (routing_enums_pb2.LocalSearchMetaheuristic.GUIDED_LOCAL_SEARCH)
    search_parameters.time_limit.FromSeconds(1)

    # Solve the problem.
    solution = routing.SolveWithParameters(search_parameters)
    # Print solution on console.
    if solution:
        print_solution(data, manager, routing, solution)


if __name__ == '__main__':
    main()

The answer I am getting is:

Objective: 27400 Route for vehicle 0: 0 Load(0) -> 1 Load(1) -> 2 Load(0) -> 3 Load(1) -> 6 Load(0) -> 5 Load(1) -> 4 Load(0) -> 0 Load(0) Distance of the route: 27400m Load of the route: 0

1. So, first travel is OK. What I picked on 1 was left on 2.
2. Then pick something on 3 but unloaded on 6 (also 2!). This package was needed at 4!.
3. Then load at 5 (there was no package here to be loaded) and unload at the base (there was nothing to deliver here).

Can anyone point me in the right direction? What variable am I missing? What is going wrong? Is my data["demands"] ok?