I’ve got Hass.io installed with the Mosquito MQTT broker (thx to the addon store), and the integration activated (with the option to automatically add entities).
I added a Shelly Plug S (https://shelly-api-docs.shelly.cloud/#mqtt-support) to my setup, and configured it to send MQTT stuff to the Mosquito.
Configuration is set without retain flags on the Shelly plug.
I can see the MQTT info coming in when subscribing to the # topic.
Unfortunately, I don’t see anything being added to HA in devices - I’ve had to manually add them (sensors/binary_sensors/switches) to get them working through HA - am I missing something?