Upstream test: testCheckInterceptedMQTTMessageProperties] [https://github.com/michalxo/activemq-artemis/commit/119e90c6c5bd2ae523e01368544ae3cb66987c4a#diff-d2a66663368a8238bae6dc85ccbd5a5d|org/apache/activemq/artemis/tests/integration/interceptors/MQTTInterceptorPropertiesTest.java
Asked for PR.
While implementing an upstream test and checking MQTT Outgoing interceptor, I have noticed that retain flag set to true is being lost when sending message out.
Also the subscribing client can identify if a received message was a retained message or not, because the broker sends out retained messages with the retained flag still set to true.
So after all this might be a matter of implementation. (feature enhancement or rejected bug)