-r, --retain
If retain is given, the message will be retained as a “last known good” value on the broker.
When a message is published to the configuration topic, the thing appears in the inbox, once you add or it auto-approves the thing, the channel should appear. This is done by OH by subscribing to the configuration topic to get the channel from state_topic.
When a message is published without the -r option, the message is deleted from the MQTT configuration topic as soon as it appears in the inbox, and therefore OH cannot find the message in the configuration topic to look for the channel details from state_topic.