This is verly likely because we had to disable uplink fields on MQTT because those were causing a lot of trouble. We intend to re-enable them when we have found a solution for the underlying problem, but that may take some time.
To work around this, you can empty the “Field” of your “ttn uplink” node, and use a function to select the fields you want from of the object that you will then get out of the “ttn uplink” node.