"A Server MUST store the message in accordance to its QoS properties and ensure onward delivery to applicable subscribers [MQTT-4.3.2-2]"
This requirement is rather vague - should be more precise, as the rules for storing a message are nowhere explicitly stated.
Isn't storage rules not just depending on QoS: but also depending on RETAIN flag?
Also the statement seems to contradict
MQTT-188.8.131.52 that states that any "data" must be stored for the life of a connection.