Nested field type will only search as one doc count in elastic search

I am trying to use AggregationBuilders to aggregate my document count by its client name since we have multiple clients associated with a single doc. we are using the nested field client_name to index and search for a document.

Here is my data format

{
    "id": "5f68c6c80f52a45a0db6d470",
    "name": "Mi Note 10 Lite",
    "brand": {
      "name": "xiaomi"
    },
    "client_name": [
      {
        "name": "client a"
      },
      {
        "name": "client b"
      }
    ]
  }

when I'm trying to search with aggregation filter for a client a and b, it will be only return one document count

"aggregations": {
      "client a": {
         "doc_count": 1
     },
     "client b": {
         "doc_count": 0
     },
}

I am using java API with elastic search 6.3.

here is my query code

boolQuery.should(new NestedQueryBuilder(CLIENT_NAME,
                        QueryBuilders
                                .termQuery(CLIENT_NAME + "." + NAME, token),ScoreMode.None));

and here is my AggregationBuilders.filter(k,v);

should not aggregation bucket return doc count 1 on both bucket or am I missing something?

Adding a working example with index data, mapping, search result, and search query

Index Mapping:

    {
  "mappings": {
    "properties": {
      "client_name": {
        "type": "nested",
        "properties": {
          "name": {
            "type": "keyword"
          }
        }
      }
    }
  }
}

Index Data:

{
    "id": "5f68c6c80f52a45a0db6d470",
    "name": "Mi Note 10 Lite",
    "brand": {
      "name": "xiaomi"
    },
    "client_name": [
      {
        "name": "client a"
      },
      {
        "name": "client b"
      }
    ]
  }

Search Query:

{
    "size" : 0,
	"aggs": {
		"client": {
			"nested": {
				"path": "client_name"
			},
			"aggs": {
				"client_count": {
					"terms": {
						"field": "client_name.name"
					}
				}
			}
		}
	}
}

Search Result:

"aggregations": {
"client": {
  "doc_count": 2,
  "client_count": {
    "doc_count_error_upper_bound": 0,
    "sum_other_doc_count": 0,
    "buckets": [
      {
        "key": "client a",
        "doc_count": 1
      },
      {
        "key": "client b",
        "doc_count": 1
      }
    ]
  }
}

You can refer this SO answer, to convert the above elasticsearch query to JAVA code.