Wildcard and phrase_prefix in _all field by multi_match query

Elastic Stack Elasticsearch
1

I want to match both by wildcard and phrase_prefix in the _all field. But the queries below don't work for me.

{'query': {'multi_match': {'query': 'demen*', 'type': 'phrase_prefix', 'fields': ['_all']}}}

{'query': {'multi_match': {'query': 'demen', 'type': 'phrase_prefix', 'fields': ['_all']}}}

The queries above work if I specify fileds.

{'query': {'multi_match': {'query': 'demen*', 'type': 'phrase_prefix', 'fields': ['Diag', 'Text']}}}

{'query': {'multi_match': {'query': 'demen', 'type': 'phrase_prefix', 'fields': ['Diag', 'Text']}}}

My mappings:

...
"type": {
  "_all": {
    "analyzer": "stem_analyzer"
  },
  "properties": {
    "field" : {
      "type": "string",
      "analyzer": "stem_analyzer"
    }
  }
}
...
"settings" : {
  "index" : {
    "creation_date" : "1470145125821",
    "analysis" : {
      "filter" : {
        "stem_possessive_filter" : {
          "name" : "possessive_english",
          "type" : "stemmer"
        }
      },
      "analyzer" : {
        "stem_analyzer" : {
          "filter" : [ "standard", "lowercase", "stem_possessive_filter" ],
          "tokenizer" : "standard"
        }
      }
    },
    "number_of_shards" : "5",
    "number_of_replicas" : "1",
    "uuid" : "vKKmroWnTtS7JCIz6Zk6Ew",
    "version" : {
      "created" : "2030399"
    }
  }
}

Why wildcard and phrase_prefix don't work with the _all field? How can I make them work?