Synonym graph on shingles gives error; works with lenient=true

[chari (chari madabhushi)]

I get an error :-
RequestError(400, 'illegal_argument_exception', 'failed to build synonyms')
with the following analyzer, but it works if I set lenient=true. Any insight into why I get an error and if there is any downside to the use of lenient=true ?

es.indices.create(index= "myIndex", body=  {
    "settings": {
      "number_of_shards": 1,
      "analysis": {
         "filter": {
           "shingle_filter":{
              "type" : "shingle",
              "max_shingle_size" : 2,
              "min_shingle_size" : 2,
              "output_unigrams" : "false",
              "output_unigrams_if_no_shingles" : "true",
              "enable_position_increments":"false"
           },
           "kill_filler": {
              "type": "pattern_replace",
              "pattern": ".*_.*",
              "replace": "" 
           },
           "graph_synonyms" : {
                        "lenient": "true",        #### error if I remove this 
                        "type" : "synonym_graph",
                        "synonyms_path" : "synonyms.txt"
            },
         },
        "analyzer": {
          "shingle_analyzer": {
            "tokenizer": "standard",
              "filter": [
                "lowercase",
                "stop",
                "graph_synonyms",
                "shingle_filter",
                "kill_filler",
                
              ]
          }
        },  
      }
   },
  "mappings": {
        "properties": {
          "content": {
            "type": "text",
            "fields": {
                "shingles": {
                    "type": "text",
                    "analyzer": "shingle_analyzer"
                }
            }
          },
          "my_join_field": { 
            "type": "join",
            "relations": {
              "document": "page" 
            }
          }   
        }
  }    
})