I have observed that new MQTT subscriptions on a given topic, receive older retained messages. Instead of getting the latest retained message published on the topic, the new subscription received an older message, published before that last one.
I have created a mqtt-test project that demonstrated the problem. Check the readme, and run the Artemis broker test:
mvn -Dtest=ArtemisTest verify
The test project contains multiple MQTT brokers tests. Only Artemis broker tests fail.