Before changing a brightness widget, compare it with the message WLED sends. This check is for an installation already connected to an MQTT broker.
Copy the device topic from WLED’s MQTT settings. The documented brightness topic is that prefix followed by /g, with a value from 0 to 255. Open a second MQTT client, inspect that exact topic, then change brightness in WLED’s normal interface and look for the new value.
If a fresh value arrives but the widget disagrees, check the widget’s topic and scale. If no update arrives, check the MQTT connection and topic configuration first. Also, /v is documented as XML status, not JSON; pointing a JSON parser at it is a separate problem.
AirMQTT is Stargap’s phone client for viewing topic messages and their history: AirMQTT | MQTT Client & iCloud Sync. Another MQTT client can make the same comparison. The topic names and payload format are in WLED’s MQTT reference: MQTT - WLED Project