Thank you, @jstuardo, for bringing this issue to our attention, and @Brits for your time and effort in troubleshooting it.

The issue was caused by the CONNACK response incorrectly setting Max QoS to 2, which was not compliant with the MQTT specification, leading MQTTnet to reject the connection. We have now resolved this. The Public FREE MQTT Broker has been updated with the fix. However the downloadable version will take little more time to update as we are in the mid of functional integration.

We highly appreciate your feedback and are happy to assist you always.