I can't subscribe to MQTT topic referring to single field

Application Development
2

You did right, however, https://status.thethings.network shows:

Uplink Fields on MQTT Disabled

Identified - For performance reasons we unfortunately had to disable publishing Uplink Fields to MQTT in our EU region. This means that the values from payload decodes are no longer published to individual [AppID]/devices/[DevID]/up/[Field] topics. Until this is resolved, you can subscribe to [AppID]/devices/[DevID]/up and read the payload_fields field from the uplinks you receive there.
Nov 18, 11:00 CET

See also MQTT used to work fine, but stopped getting data and TTN uplink node Node-Red connected but not working.