Template problem: it has the non-numeric value: unknown

I’m using a template sensor (aussenthermometer_maximum) to save a state of another sensor (aussenthermometer_max_letzte_24_stunden) and enrich with an attribute with an attribute.

Source is a statistic sensor, which is calculation out of another sensor, which is sometime not available a startup of HA.

Then I thought to check against a number and use the old value if it is not a number. But as only trigger sensors restore their state during restart, I converted to trigger sensor as well. But still get the error on reboot:

ValueError: Sensor sensor.aussenthermometer_maximum has device class temperature, state class measurement unit °C and suggested precision None thus indicating it has a numeric value; however, it has the non-numeric value: unknown (<class ‘str’>)

But I don’t understand why this is not working, as it should check if it is available and returns a number and only then take this into the sensor. Otherwise take the old state or the restored state before reboot.

  - trigger:
      - platform: state
        entity_id: sensor.aussenthermometer_max_letzte_24_stunden
      - platform: homeassistant
        event: start
      - platform: event
        event_type: event_template_reloaded
  - sensor:
      - name: Außenthermometer Maximum
        unique_id: unique_id_template_sensor_aussenthermometer_maximum
        state: >
          {% if is_number(states('sensor.aussenthermometer_max_letzte_24_stunden')) %}
            {{ states('sensor.aussenthermometer_max_letzte_24_stunden') }}
          {% else %}
            {{ states('sensor.aussenthermometer_maximum') }}
          {% endif %}
        icon: mdi:thermometer-plus
        device_class: temperature
        state_class: measurement
        unit_of_measurement: "°C"
          zeit: >
            {{ states('sensor.aussenthermometer_max_letzte_24_stunden_zeit') }}

As this is coming from https://github.com/home-assistant/core/pull/85605 I found further problems like this on githup.

So any ideas why the trigger-approach is not working and still gives the error on startup?

Or any idea or example to define a template now, which is not giving such errors on startup?