The usual way of doing this is to have your plantgateway publish its message with the retain flag set. The broker will then save the message and send it whenever a client subscribes to the topic, such as when HA restarts.
The last I heard, the embedded broker does not support this, so you must be using mosquitto or another external broker that does.
If you connect using another client, you can see if mosquitto has stored the value correctly (it may not be configured with a database). I generally do that with
mosquitto_sub -t "#" -v
You may need to add host, username and password parameters.
This will print out all message topics and the values that have been retained in the broker’s database.
Also, if you can paste the mosquitto configuration file you are using, that will say if it has the database set up.
I’m not too sure how hassio works, as I don’t use it, but when you restart HA, does the broker restart as well?
Its just that I notice your mosquitto configuration does not set up a file to store retained messages, so if the broker restarts, it will have lost all its retained messages.
I can’t see any other problems with your configuration.
I tried that now. Even after a reboot I can get the last value using mqttbox. I don’t know if mosquitto also reboots (don’t think so), but either it does or not, the data is still here after the reboot. However, it still does not show up in the plant section.
While writing this I came up with the idea to look at the different sensors that are used by the plant component. So for example I can see the value of sensor.limette_temperatur in ha (after reboot). And it gets updated right after the reboot.