How can this binary_sensor be unknown?

Configuration
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2

The Template Binary Sensor’s template is evaluated when binary_sensor.smoke_1 changes state. Until that happens, the Template Binary Sensor’s initial value is unknown.

For your application, the binary_sensor is a smoke detector so its state rarely ever changes (hopefully never, otherwise it has detected smoke). However, that makes it an entity that’s a bit too inert/quiet for use in a Template Binary Sensor.

I suggest creating a Trigger-based Template Binary Sensor with a State Trigger, to monitor the smoke detector, and an Event Trigger, to detect when Template entities are reloaded.

template:
  - trigger:
      - platform: state
        entity_id: binary_sensor.smoke_1
      - platform: event
        event_type: event_template_reloaded
    binary_sensor:
      - name: fumee_rdc
        device_class: smoke
        state: "{{ is_state('binary_sensor.smoke_1', 'on') }}"

After you create it, execute Developer Tools > YAML> Reload Template Entities. Its initial value should now be the same as binary_sensor.smoke_1.

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