MQTT: Split does not work

at the moment I am a bit clueless as I am unable to use a Split method in the MQTT configuration .
I receive a string from a sensor

- name: "AHT25_5_status"
      state_topic: "AHT25-5/version"

This results in for example “aht25MQTT.ino,, Huber1, -80dbm,” and I can easily extract the last component “-80dbm” in ->developer tools → template with
{{ states('sensor.aht25_5_status').split(',')[3]}}
But if I try the same within mqtt.yaml with

    - name: "AHT25_5_RSSI"
      state_topic: "AHT25-5/version"
      value_template: >
       {{ value.split(',')[3] }}

I always get ‘unknown’ as result for AHT25_5_RSSI
I can, however, use successfully

    - name: "AHT25_5_SSID"
      state_topic: "AHT25-5/version"
      value_template: >
       {{ value.split(',')[2] }}

Any idea why?

I had the line ’ unit_of_measurement: dbm’ in the MQTT definition and the text ‘-81dbm’ was not numeric.