es5.6和7.6关于高亮的语法差别？

相同的mappings和查询语句，在7.6版本上可以返回高亮字段，在5.6版本上无法返回高亮字段，想问问大佬，如何在5.6上返回高亮字段，是语法上的差别还是5.6就不支持？

# 索引创建语句

PUT example/docs/1

{

    "region": "CN",

    "manager": {

        "age": 22,

        "name": {

            "first": "李",

            "last": "小明"

        }

    }

}

# 查询语句

GET example/_search

{



  "highlight": {

    "fields": {

      "manager.name.last": {},

      "manager.name.first": {}

    }

  }, 

  "query": {

    "query_string": {

      "query": "李小明"

    }

  }

}

# es7.6返回结果，有高亮字段

{

  "took" : 2,

  "timed_out" : false,

  "_shards" : {

    "total" : 1,

    "successful" : 1,

    "skipped" : 0,

    "failed" : 0

  },

  "hits" : {

    "total" : {

      "value" : 1,

      "relation" : "eq"

    },

    "max_score" : 0.5753642,

    "hits" : [

      {

        "_index" : "example",

        "_type" : "docs",

        "_id" : "1",

        "_score" : 0.5753642,

        "_source" : {

          "region" : "CN",

          "manager" : {

            "age" : 22,

            "name" : {

              "first" : "李",

              "last" : "小明"

            }

          }

        },

        "highlight" : {

          "manager.name.first" : [

            "<em>李</em>"

          ],

          "manager.name.last" : [

            "<em>小</em><em>明</em>"

          ]

        }

      }

    ]

  }

}

# es5.6返回结果，无高亮字段

{

  "took": 2,

  "timed_out": false,

  "_shards": {

    "total": 5,

    "successful": 5,

    "skipped": 0,

    "failed": 0

  },

  "hits": {

    "total": 1,

    "max_score": 0.84748024,

    "hits": [

      {

        "_index": "example",

        "_type": "docs",

        "_id": "1",

        "_score": 0.84748024,

        "_source": {

          "region": "CN",

          "manager": {

            "age": 22,

            "name": {

              "first": "李",

              "last": "小明"

            }

          }

        }

      }

    ]

  }

}