doc['objectValue'][3]取值问题

es版本7.9
存储的数据格式如下：
 {
                "_index": "properties_trsq_rhqsqsq_2021-11",
                "_type": "_doc",
                "_id": "3cae2cb0be59a5b44af5eb7d5ce945dd",
                "_score": 1.0,
                "_source": {
                    "productId": "trsq_rhqsqsq",
                    "objectValue": [
                        11,
                        1232,
                        3,
                        4,
                        6
                    ],
                    "type": "array",
                    "deviceId": "868739057460077",
                    "formatValue": "[null,\"13.10℃\",\"12.40℃\",\"12.90℃\",\"12.70℃\"]",
                    "propertyName": "温度",
                    "createTime": 1636878706315,
                    "property": "TEMP",
                    "id": "3cae2cb0be59a5b44af5eb7d5ce945dd",
                    "value": "[null,13.1,12.4,12.9,12.7]",
                    "timestamp": 1636878701314
                }
            },
想要实现的效果：
对objectValue字段指定下标的数据进行聚合操作。
问题：
通过doc['objectValue'][3]获取出来的值，是通过排序了的。不是原始的数据顺序。

查询聚合语句如下：
{
    "aggs": {
        "avg_grade": {
            "avg": {
                "script": "doc['objectValue'][3]"
            }
        }
    }
}
期望结果：
 "aggregations": {
        "avg_grade": {
            "value": 4
        }
    }
返回结果：
 "aggregations": {
        "avg_grade": {
            "value": 33.0
        }
    }