PerFieldSimilarity for synonym expansion at query time

Elastic Stack Elasticsearch
2

I get the same score for all documents when I run the following:

DELETE index

PUT index 
{
  "settings": {
    "number_of_shards": 1, 
    "analysis": {
      "filter": {
        "my_syns": {
          "type": "synonym",
          "synonyms" : [
            "shirt,blouse"
          ]
        }
      },
      "analyzer": {
        "my_index_analyzer": {
          "type": "custom",
          "tokenizer": "whitespace"
        },
        "my_search_analyzer": {
          "type": "custom",
          "tokenizer": "whitespace",
          "filter": ["my_syns"]
        }
      }
    }
  },
  "mappings": {
    "doc": {
      "properties": {
        "field": {
          "type": "text",
          "analyzer": "my_index_analyzer",
          "search_analyzer": "my_search_analyzer"
        }
      }
    }
  }
}

PUT index/doc/1
{
  "field": "shirt xyz"
}

PUT index/doc/2
{
  "field": "blouse xyz"
}

GET index/_search
{
  "query": {
    "match": {
      "field": "shirt"
    }
  }
}