Unit conversion causes self-referencing sensors to break

[mekaneck]

Let's define a trigger-based template sensor which reports the max temperature from all time:

template:
  - trigger:
      - platform: state
        entity_id: sensor.outside_temperature_celsius
    sensor:
      - name: "Max Outside Temperature"
        unit_of_measurement: "°C"
        device_class: temperature
        state: "{{ max(this.state | float(0), trigger.to_state.state | float(0) }}"

If this template sensor stays in °C, there is no issue. However if the units are changed to °F from the UI, we now have a scenario where the undesired value is selected by the max function (because this.state has already been converted to °F and is therefore larger). Even worse, that returned value is again "converted" to °F. The end result is that upon each trigger, the value of this template sensor skyrockets due to double-conversion.

This unintended behavior will occur for any sensor which:

• is defined with a device_class that supports unit conversions
• contains a template which self-references
• has its unit of measurement changed, either by a user from the UI or automatically by HA due to the unit system selected when the sensor was added.