Resolution:
Messages with the persistent delivery mode are always written to persistent storage, except when they are published to a topic that has no durable subscribers. When a topic has no durable subscribers, there are no subscribers that need messages resent in the event of a server failure. Therefore, messages do not need to be saved.

When subscribers use FT URLs (a comma separated list), the behavior is slightly changed. Indeed, persistent messages are now persisted on disk, even if there are only non-durable subscribers. The reason is that in case of server failure, once the server is restarted and if the EMS clients FT configuration are such that the number of reconnect attempts allowed them to reconnect, the EMS clients have been logically always connected and therefore the server must deliver persistent messages to those subscribers.