Not understanding the behavior of _id on ES - elasticsearch

Question

I have documents in a Mongo database, they use the _id field as an index.
I use Monstache to sync ES with Mongo's op log, so the documents in ES have the same _id field.
When searching a specific document, Kibana shows:
Tags:
tag1 testtag CreatedOn:
October 26th 2018, 14:25:57.053
_id:
FRaqDPIzWcVI2dl-oA9uUFHLVFQk8qIqqhySWSkM7Ds
_type:
testobject
_index:
test.object
_score:
0
but then a query with Nest, returns this in the Documents array:
_id = 0d5aa177-3066-4c6a-aaf5-9b887ae7297f
and when I look in the Hits array, I see:
Id = FRaqDPIzWcVI2dl-oA9uUFHLVFQk8qIqqhySWSkM7Ds
So in Documents, the _id is now an unrelated guid, but in Hits the _id is called Id and it has the right value.
Why is that, and is there a way to get the proper value for _id in Documents?
Edit: more info
This is the object; since it's shared by MongoDB and ES, it has attributes for both.
[Nest.ElasticsearchType, BsonIgnoreExtraElements]
public class TestObject
{
public string _id { get; set; }
public string OwnerId { get; set; }
public Flags Flags { get; set; }
[Nest.Text, BsonIgnoreIfDefault] public string Title { get; set; }
[Nest.Text] public string Tags { get; set; }
[Nest.Ignore] public string Hash { get; set; }
[Nest.Ignore, BsonIgnoreIfDefault] public string Link { get; set; }
}
This is the code creating the index:
private static void InitializeElasticSearch(string ConnectionString)
{
var Settings = new ConnectionSettings(new Uri(ConnectionString))
.DefaultIndex(_IndexName)
.DefaultFieldNameInferrer(_ => _)
.DefaultMappingFor<TestObject>(_ => _.Ignore(I => I._id));
_ElasticClient = new ElasticClient(Settings);
if (!_ElasticClient.IndexExists(_IndexName).Exists)
{
// create the index
var CreateIndexResponse = _ElasticClient.CreateIndex(_IndexName, C => C
.Settings(S => S
.Analysis(A => A
.CharFilters(Cf => Cf
.Mapping("expressions", E => E
.Mappings(TextLists.Expressions)
)
)
.TokenFilters(Tf => Tf
.Synonym("synonyms", Sy => Sy
.Synonyms(TextLists.Synonyms)
.Tokenizer("whitespace")
)
)
.Analyzers(An => An
.Custom("index", Ca => Ca
.CharFilters("expressions")
.Tokenizer("standard")
.Filters("standard", "synonyms", "stop")
)
)
)
)
.Mappings(M => M
.Map<TestObject>(Mm => Mm
.AutoMap()
.Properties(P => P
.Text(T => T
.Name(N => N.Title)
.Analyzer("index")
)
.Text(T => T
.Name(N => N.Tags)
.Analyzer("index")
)
)
)
)
);
Then, the query code:
var R = await _ElasticClient.SearchAsync<TestObject>(Sr => Sr
.Query(Q =>
{
// do we query 'all' ?
if (Terms == "*") return Q.MatchAll();
// or do we have a general query
return Q
.MultiMatch(Fu => Fu
.Fields(F => F
.Field(Ff => Ff.Tags)
.Field(Ff => Ff.Title)
)
.Query(Terms)
.Fuzziness(Fuzziness.EditDistance(2))
);
})
.Take(_MaxObjectReturned)
);