Try this instead:

        "bool":{
          "must_not":[
            {
              "nested":{
                "path":"fixed_fields",
                "filter":{
                  "term":{
                    "fixed_fields.id":38
                  }
                }
              }
            }
          ]
        }

Try this instead:

{
 "bool": {
    "must_not": [{
        "nested": {
            "path": "fixed_fields",
            "filter": {
                "term": {
                    "fixed_fields.id": 38
                }
            }
        }
    }]
  }
}