_class not inserted when using with Custom Converter

Hi,

I'm not quite sure if this is a bug, or I missed out certain configuration.

I have a reproduce ready at Github

In summary, I have a document

@TypeAlias("cinfo")
@Document("cinfo")
public class CInfo implements ReferenceAble {}

Where I register the converter

@Configuration
public class MongoConfig {
    @Bean
    MongoCustomConversions conversions() {
        return new MongoCustomConversions(List.of(new ReferenceAbleConverter()));
    }
}

The converter is just a simple @WritingConverter

@WritingConverter
public class ReferenceAbleConverter implements Converter<ReferenceAble, DocumentPointer<Document>> {

    @Override
    public DocumentPointer<Document> convert(ReferenceAble source) {
        return source::toReference;
    }

}

When I run the test

@DataMongoTest(excludeAutoConfiguration = EmbeddedMongoAutoConfiguration.class)
@Import(MongoConfig.class)
public class DocRefTests {
    @Autowired
    private CInfoRepository cInfoRepository;

    @Test
    void test1() {
        CInfo cInfo = CInfo.builder().cinfo("cinfo").build();
        CInfo createdCInfo = this.cInfoRepository.insert(cInfo);

        log.info("{}", createdCInfo);
    }
}

The log shows the following

2022-11-14 22:53:21.837 DEBUG 9512 --- [           main] o.s.data.mongodb.core.MongoTemplate      : Inserting Document containing fields: [cinfo] in collection: cinfo

{
    _id: ObjectId('6372531b8e27062bdd80046f'),
    cinfo: 'cinfo'
}

But if I don't import MongoConfig.class, then the output would be

2022-11-14 22:54:05.211 DEBUG 28056 --- [           main] o.s.data.mongodb.core.MongoTemplate      : Inserting Document containing fields: [cinfo, _class] in collection: cinfo

{
    _id: ObjectId('63725335c2a1c4466a4fdf38'),
    cinfo: 'cinfo',
    _class: 'cinfo'
}

Notice that _class is not being inserted to the document in the first test

Thanks!