Using Json format in Confluent Platform Schema Registry?

Question

I'm trying to put several event types in the same Kafka topic using the JSON format, but in the Producer implementation I'm always getting org.apache.kafka.common.errors.SerializationException: Error serializing JSON message. Seems that the annotation @Schema isn't working as expected is like the schema defined by the annotation isn't enriched properly and in the method that validates the backward compatibility the schema defined by my event has the schemaObj empty and the result is not compatible and fails.

My event:

@Schema(
  value = "1",
  refs = Array(new SchemaReference(name = "event", subject = "event"))
)
case class Event(@BeanProperty id: String,
                  @BeanProperty name: String)

Producer:

def send(): Unit = {
    val props = new Properties() {
      put(ProducerConfig.BOOTSTRAP_SERVERS_CONFIG, "localhost:9092")
      put(
        ProducerConfig.KEY_SERIALIZER_CLASS_CONFIG,
        "org.apache.kafka.common.serialization.StringSerializer"
      )
      put(
        ProducerConfig.VALUE_SERIALIZER_CLASS_CONFIG,
        "io.confluent.kafka.serializers.json.KafkaJsonSchemaSerializer"
      )
      put("auto.register.schemas", "false")
      put("use.latest.version", "true")
      put("schema.registry.url", "http://127.0.0.1:8081")
      put("json.fail.invalid.schema", "true") 
    }

    val producer = new KafkaProducer[String, Event](props)

    val topic = "all-json"
    val key = "key1"
    val event = Event("id", "name")

    val record = new ProducerRecord[String, Event](topic, key, event)
    producer.send(record).get
}

By the command line, I can perfectly produce the events. The JSON Schema is modeled by

{
  "oneOf": [
     { "$ref": "Event.schema.json" },
     { "$ref": "EventB.schema.json" }
  ]
}

...

the dependencies of confluent used are the version 6.0.1.

Do you know what is the issue?