搜索结果未能高亮搜索词 (BM25)

ElasticSearch Version: 6.5.4
运行环境：MacOS Mojave

场景：我在使用内容 吉林期中试题  进行搜索时同批次结果中有的结果对 包含 吉林、期中、试题 的标题进行了高亮 有的却没有全部高亮 因为考虑词频问题 title 在创建索引时 similarity 指定使用了 BM25 ，在未指定使用BM25的默认情况下 未出现该问题

结果如下所示




 
请求的DSL：

{

  "_source": [

    "id",

    "stage",

    "subject_id",

    "title"

  ],

  "size": 20,

  "query": {

    "function_score": {

      "query": {

        "bool": {

          "must": [

            {

              "multi_match": {

                "query": "吉林期中试题",

                "fields": [

                  "title^3"

                ],

                "tie_breaker": 0.3,

                "type": "best_fields",

                "minimum_should_match": "90%"

              }

            },

            {

              "terms": {

                "flag": [

                  7,

                  8,

                  17,

                  19

                ]

              }

            }

          ]

        }

      },

      "boost_mode": "multiply",

      "functions": [

        {

          "exp": {

            "created_at": {

              "origin": "now",

              "scale": "3650d",

              "decay": 0.3

            }

          }

        }

      ]

    }

  },

  "highlight": {

    "pre_tags": "<em>",

    "post_tags": "</em>",

    "encoder": "html",

    "fields": {

      "title": {

        "fragment_size": 500,

        "number_of_fragments": 1

      },

      "intro": {

        "fragment_size": 500,

        "number_of_fragments": 1

      }

    }

  }

}