Elasticsearch Java API posting JSONObject

I have an elasticsearch 8.1 server running to which I want to post JSONObjects. I build my IndexRequest using the Java API as follows:

JSONObject asset = (JSONObject) a;
IndexRequest<JSONObject> req;
req = IndexRequest.of(b->b.index("assets").document(asset).id(asset.getString("symbol")));
ESClient.index(req)

The ESClient is built like this:

    public ElasticsearchClient client() {
      RestClient restClient = RestClient.builder(
            new HttpHost("localhost", 9200)).build();

      ElasticsearchTransport transport = new RestClientTransport(
            restClient, new JacksonJsonpMapper());

      return new ElasticsearchClient(transport);
}

The JSONObject (org.json.JSONObject) asset is quite big, and it is definitely not empty. When I query the index, the relevant part of the result look like this:

    "hits" : 
      [
        {
          "_index" : "assets",
          "_id" : "BTC",
          "_score" : 1.0,
          "_source" : {
            "empty" : false
          }
        }
      ]

It looks as if the document got uploaded, but nothing of the body. What am I doing wrong here? All help appreciated.

EDIT: It works when writing the JSONObject to a file and posting a FileReader of that file, but that is in no way a solution to my problem:

   JSONObject asset = (JSONObject) a;
   File file = new File("asset");
   FileWriter fileWriter = new FileWriter(file);
   fileWriter.append(asset.toString());
   fileWriter.close();
   FileReader fileReader = new FileReader("asset");
   IndexRequest<JsonData> req;
   req = IndexRequest.of(b->b.index("assets").withJson(fileReader).id(asset.getString("symbol")));
   fileReader.close();
   ESClient.index(req)