Kotlin class discriminator with kotlinx serialization

Hello, we are using the Mongo kotlin library to serialize a hierarchy of objects. These are sealed classes and we want to define a specific discriminator for these objects.

@OptIn(ExperimentalSerializationApi::class)
@JsonClassDiscriminator("type")
@Serializable
sealed class TestInterface(
  @Contextual val _id: ObjectId
)

@Serializable
@SerialName("TestClass1")
class TestClass1 : TestInterface(ObjectId())

@Serializable
@SerialName("TestClass2")
class TestClass2 : TestInterface(ObjectId())

When inserting instances of these objects in a collection, a discriminator field _t is written instead of type.
According to the library code, the BsonEncoder class does not seem to support the kotlinx annotation and only looks at the global configuration:

is PolymorphicKind -> {
    writer.writeStartDocument()
    writer.writeName(configuration.classDiscriminator)
    isPolymorphic = true
}

Is this analysis correct ? is there a way to define a specific discriminator for our objects ? possibly without using the kotlinx serialization ?

Thanks