Hello Dan,
I had the same problem with MQTT auto-discovery and found out that it has to do with the broker’s subscription limit. I was trying to use the EMQX serverless broker, which has a limit of 10 subscriptions per client. As a result, the MQTT auto-discovery does not work, since it subscribes to dozen of topics, and even if you disable auto-discovery and configure the sensors manually in the configuration.yaml, you still have the 10 subscriptions limit (which ends up limiting your system to 10 sensors).
I solved my problem changing to the free version of the HiveMQ Cloud. Apparently there is not subscription limit per client (I could not find any explicit information about that). As a result, I’m now able to use the MQTT auto-discovery and all my sensors are working fine. I was glad for this solution, since I do not have to lose time typing the sensors manually in the configuration.yaml.
Best regards.