This message is currently suppressed (as per the commit). This message means that a device has reconnected, which is a frequent case. This message may need to pay attention only if a device is in a reconnection loop (you've apparently met that issue, reboot the device to fix). The repeating warning may flood logs quickly and cause more serious failures when the storage is full. So we decided to suppress it.
In the future, we will deprecate MQTT. Consider switching to HTTP polling to avoid issues like this.