about emqx qos 2

i used org.eclipse.paho.client.mqttv3[1.2.5] about QoS 2 transmission action explain https://www.emqx.com/en/blog/introduction-to-mqtt-qos it seems like after the client received PUBREC it will delete the message, but the [eclipse.paho qos] source code does not delete message action it seems like below and why it did not deleted the message

ClientState.notifyReceivedAck
```java
else if (ack instanceof MqttPubRec) {
// Complete the QoS 2 flow. Unlike all other
// flows, QoS is a 2 phase flow. The second phase sends a
// PUBREL - the operation is not complete until a PUBCOMP
// is received
MqttPubRel rel = new MqttPubRel((MqttPubRec) ack);
this.send(rel, token);
}
```

ClientState.send
```java
if (message instanceof MqttPingReq) {
this.pingCommand = message;
}
else if (message instanceof MqttPubRel) {
outboundQoS2.put( Integer.valueOf(message.getMessageId()), message);
persistence.put(getSendConfirmPersistenceKey(message), (MqttPubRel) message);
}
else if (message instanceof MqttPubComp) {
persistence.remove(getReceivedPersistenceKey(message));
}
```

Start by reading ClientState.notifyReceivedAck and ClientState.send, focusing on the MqttPubRec, MqttPubRel, and MqttPubComp branches and the persistence keys shown in the report. Compare that lifecycle with the MQTT QoS 2 specification; done means documenting or reproducing the message-removal behavior and identifying whether the implementation is incorrect.

Written by the indexing model from the issue text.