When I subscribe an MQTT client to a Universal Messaging broker with the multi-level wildcard topic filter “#”, then the broker does not behave as expected. I’ll only receive messages for topics that have already existed before. I won’t receive messages that will be published to new topics. I’ll only be able to listen to the new topics, if I subscribe again after these topics have been used for the first time. The expected behavior is that I’ll receive any message – no matter whether the topic has been used in the past or whether the topic is new.
This issue does not exist with the single-level wildcard “+”.
Reference: MQTT specification regarding “Topic Names and Topic Filters” MQTT Version 3.1.1