Automation for switch off inverter does not trigger

Hi guys,
I have a quick question about an automation.
I want an inverter to be switched off if the power consumption is below 21 watts for at least 1 minute.
I have an sensor entity ("multiplus[…]) in Home Assistant that contains the watt value as a number, i.e. without a unit. (E.g. “35”).
I also have a selection entity “vebus mode[…]” in which the inverter mode can be changed (ON, OFF, etc.)
Now I have written an automation that does not trigger, even though the watt value remains below 21 watts for a minute.

What am I doing wrong?

alias: Wechselrichter deaktivieren
description: ""
trigger:
  - platform: numeric_state
    entity_id:
      - sensor.multiplus_l1_out_template
    for:
      hours: 0
      minutes: 1
      seconds: 0
    below: 21
condition:
  - condition: state
    entity_id: select.victron_vebus_mode_239
    state: "ON"
action:
  - service: select.select_option
    metadata: {}
    data:
      option: "OFF"
    target:
      entity_id: select.victron_vebus_mode_239
mode: single

The value needs to change from above 21 to below 21 and then stay there for a minute before the trigger will fire.