Mongoose aggregatePaginate with population

I am trying to paginate an aggregation query using mongoose-aggregate-paginate
The field bookings is an array of ObjectId’s of collection “bookings”

BookingModel: model("booking", bookingSchema)

The model :

const tourGroupSchema = new Schema({
  product: String,
  date: String,
  time: String,
  bookings: [{ type: Schema.Types.ObjectId, ref: "booking" }], //need to be populated
  task: String,
  task_id: String,
  notes: String,
  notes_list: { type: Array, default: [] }, // -
  guide_id: String,
  guide_uds_id: String,
  guide_confirmation: String,
  guide_details: String,
  guides_asked: Array,
  guide_email_sent: Boolean,
  vehicle_id: String,
  index: {
    type: Number,
    default: 1,
  },
});

tourGroupSchema.plugin(mongoosePaginate);
tourGroupSchema.plugin(mongooseAggregatePaginate);


TourGroupModel: model("tour_group", tourGroupSchema),

My pipeline :

const pipeline = [];

   // Stage 1: Match documents based on product_ids
pipeline.push({
  $match: {
    product: { $in: product_ids },
       },
    });

//WHAT I HAVE TRIED

pipeline.push({
      $lookup: {
        from: "bookings",
        localField: "bookings",
        foreignField: "_id",
        as: "bookings",
      },
    });

const aggregation = await TourGroupModel.aggregate(pipeline);
const data = await TourGroupModel.aggregatePaginate(aggregation, {
  page: page,
  limit: 25,
  sort: { date: 1, time: 1, index: 1 },
        });

result :

    {
        "docs": [
            {
                "_id": "64cc8fa5fe9011604f2dca21",
                "product": "63958eaedaf242d73c7b9719",
                "date": "2022-04-08",
                "time": "15:00",
                "bookings": [
                    "641c46c8e0edc61fec97a830",
                    "641c46c9e0edc61fec97a83a",
                    "641c46c8e0edc61fec97a835"
                ],
                "index": 1,
                "__v": 2,
                "notes_list": []
            },
...rest

As you can see the bookings array of each TourGroup dont come with populated documents but still their ids instead, any ideas why ?