Aggregation bucket results have wrong keys or are not returned at all

[marcinjahn]

NEST/Elasticsearch.Net version: 7.17.5

Elasticsearch version: 8.6.2

.NET runtime version: .NET 6.0

Operating system version: Fedora 38, elasticsearch is running in Docker

Description of the problem including expected versus actual behavior: Here's a simple query I am running:

var someDescriptor = new SearchDescriptor<MyModel>(Indices.Index(new[] { "someindex" }))
            .Aggregations(ag => ag
                .Terms("some_term_agg", term => term
                    .Field(m => m.CityCode)
                    .Size(10)));
        
var someResult = await _client.SearchAsync<PrecomputedPackage>(
    someDescriptor, 
    cancellationToken);

The corresponding REST API request JSON is as follows:

{
  "aggs": {
    "some_term_agg": {
      "terms": {
        "field": "cityCode",
        "size": 10
      }
    }
  }
}

(The above JSON was generated with _client.RequestResponseSerializer.SerializeToString(someDescriptor))

The response I'm getting with NEST contains 8 buckets:

However, each bucket is empty:

When I execute my query with elasticsearch's REST API, I get:

  "aggregations": {
    "some_term_agg": {
      "doc_count_error_upper_bound": 0,
      "sum_other_doc_count": 0,
      "buckets": [
        {
          "key": "BCN",
          "doc_count": 134
        },
        {
          "key": "ROM",
          "doc_count": 134
        },
        {
          "key": "LCJ",
          "doc_count": 132
        },
        {
          "key": "LIZ",
          "doc_count": 130
        },
        {
          "key": "WAW",
          "doc_count": 126
        },
        {
          "key": "WRO",
          "doc_count": 120
        },
        {
          "key": "BER",
          "doc_count": 115
        },
        {
          "key": "LON",
          "doc_count": 109
        }
      ]
    }
  }

(I skipped the documents, they are alright in both NEST and REST API responses)

Expected behavior I should get the same buckets values using NEST, as I do when using REST API.

This issue occurred to me while I was trying to prepare a simple reproduction example for another issue where buckets have values, but their keys are wrong - all keys have the same values equal to the name of the aggregation (e.g, "some_term_agg") instead of the value being aggreagated (like city codes).