flutterでListをpostするとサーバー側でString型になってしまう

Flutter 2.10.5
Tools • Dart 2.16.2 • DevTools 2.9.2

バックエンド
Laravel Framework 8.83.11
PHP 8.0.17

Flutterのchopperを使用してpostリクエストをしたいです。
パラメーターにList<APICustomerOrderItem> customer_order_itemsというリストがあるのですが、バックエンド側ではただのString型のInstance of 'APICustomerOrderItem'になってしまいます。
FlutterでパラメーターにListが含まれている場合どのように値を渡したら良いでしょうか？
よろしくお願いします。

Requestログ
=========HTTP Request logging=========
                baseUrl: http://localhost/api
                url: /customer_orders/create
                parameter: {customer_name: 〇〇, kana: 〇〇, send_id: 2, postal_code: null, address: null, address_second: null, phone: 〇〇, shop: 118, staff: 1, customer_order_items: [Instance of 'APICustomerOrderItem']}
                method: POST
                headers: {Authorization: 〇〇}
                body: null
                multipart: false
                parts: []
                ======================================

customer_order_service.dart
import 'package:chopper/chopper.dart';

import '../model_response.dart';
import '../../config/settings.dart';
import 'customer_order_model.dart';
import 'customer_order_model_converter.dart';
part 'customer_order_service.chopper.dart';

@ChopperApi()
abstract class CustomerOrderService extends ChopperService {
  @Post(path: '/api/customer_orders/create')
  Future<Response<Result<APICustomerOrderQuery>>> queryCustomerOrder(
    @QueryMap() Map<String, dynamic> params,
  );

  static CustomerOrderService create(String apiToken) {
    final client = ChopperClient(
      baseUrl: Settings.apiUrl,
      interceptors: [
        HttpLoggingInterceptor(),
        (Request request) async {
          return applyHeader(request, 'Authorization', 'Bearer $apiToken');
        },
        (Request request) async {
          print("""
                =========HTTP Request logging=========
                baseUrl: ${request.baseUrl}
                url: ${request.url}
                parameter: ${request.parameters}
                method: ${request.method}
                headers: ${request.headers}
                body: ${request.body}
                multipart: ${request.multipart}
                parts: ${request.parts}
                ======================================
              """);
          return request;
        },
        (Response response) async {
          print("""
                =========HTTP Response logging=========
                url: ${response.base.request!.url}
                status: ${response.statusCode}
                headers: ${response.headers}
                body: ${response.body}
                ======================================
              """);
          return response;
        },
      ],
      converter: CustomerOrderModelConverter(),
      errorConverter: const JsonConverter(),
      services: [
        _$CustomerOrderService(),
      ],
    );
    return _$CustomerOrderService(client);
  }
}

customer_order_model_converter.dart
import 'dart:convert';
import 'package:chopper/chopper.dart';

import '../model_response.dart';
import 'customer_order_model.dart';

class CustomerOrderModelConverter implements Converter {
  @override
  Request convertRequest(Request request) {
    print('request: ' + request.toString());
    final req = applyHeader(
      request,
      contentTypeKey,
      jsonHeaders,
      override: false,
    );

    return encodeJson(req);
  }

  Request encodeJson(Request request) {
    final contentType = request.headers[contentTypeKey];
    if (contentType != null && contentType.contains(jsonHeaders)) {
      print('request.body: ' + request.body.toString());
      return request.copyWith(body: json.encode(request.body));
    }
    return request;
  }

  Response<BodyType> decodeJson<BodyType, InnerType>(Response response) {
    final contentType = response.headers[contentTypeKey];
    var body = response.body;
    // print(contentType);

    if (contentType != null && contentType.contains(jsonHeaders)) {
      body = utf8.decode(response.bodyBytes);
    }
    // print('body:' + body.toString());

    try {
      final mapData = json.decode(body);
      if (mapData['status'] != null) {
        return response.copyWith<BodyType>(
            body: Error(Exception(mapData['status'])) as BodyType);
      }
      print('mapData: ' + mapData.toString());

      final apiQuery = APICustomerOrderQuery.fromJson(mapData);

      print('apiQuery:' + apiQuery.toString());
      return response.copyWith<BodyType>(body: Success(apiQuery) as BodyType);
    } catch (e) {
      chopperLogger.warning(e);
      return response.copyWith<BodyType>(
          body: Error(e as Exception) as BodyType);
    }
  }

  @override
  Response<BodyType> convertResponse<BodyType, InnerType>(Response response) {
    return decodeJson<BodyType, InnerType>(response);
  }
}

customer_order_model.dart
import 'package:json_annotation/json_annotation.dart';

part 'customer_order_model.g.dart';

@JsonSerializable()
class APICustomerOrderQuery {
  factory APICustomerOrderQuery.fromJson(Map<String, dynamic> json) =>
      _$APICustomerOrderQueryFromJson(json);

  Map<String, dynamic> toJson() => _$APICustomerOrderQueryToJson(this);

  APICustomerOrder customer_order;
  List<APICustomerOrderItem> customer_order_items;
  int status_code;
  String message;

  APICustomerOrderQuery({
    required this.customer_order,
    required this.customer_order_items,
    required this.status_code,
    required this.message,
  });
}

@JsonSerializable()
class APICustomerOrder {
  factory APICustomerOrder.fromJson(Map<String, dynamic> json) =>
      _$APICustomerOrderFromJson(json);

  Map<String, dynamic> toJson() => _$APICustomerOrderToJson(this);

  int id;
  int status;
  String customer_name;
  String kana;
  int send_id;
  String postal_code;
  String address;
  String address_second;
  String phone;
  int payment;
  DateTime deadline;
  int shop;
  int staff;
  String memo;
  DateTime created_at;
  DateTime updated_at;

  APICustomerOrder({
    required this.id,
    required this.status,
    required this.customer_name,
    required this.kana,
    required this.send_id,
    required this.postal_code,
    required this.address,
    required this.address_second,
    required this.phone,
    required this.payment,
    required this.deadline,
    required this.shop,
    required this.staff,
    required this.memo,
    required this.created_at,
    required this.updated_at,
  });
}

@JsonSerializable()
class APICustomerOrderItem {
  factory APICustomerOrderItem.fromJson(Map<String, dynamic> json) =>
      _$APICustomerOrderItemFromJson(json);

  Map<String, dynamic> toJson() => _$APICustomerOrderItemToJson(this);

  int product_id;
  int item;
  int quantity;

  APICustomerOrderItem({
    required this.product_id,
    required this.item,
    required this.quantity,
  });
}