This is a bug with windows. mdns domains do not have a concept of subdomains, so MyMqttService.mqtt.local is not a part of mqtt.local but is instead its own domain with its own IP. Windows, however, treats any domain with multiple dots as a subdomain, even mdns domains; Windows sees that mqtt.local is not a valid domain and thus gives up without even trying "MyMqttService.mqtt.local". This is unfortunately not fixable on the user end.