MQTT Pressure Conversion Weirdness

Hello, I have integrated a weather pressure sensor into home assistant using MQTT. It reports the pressure in hPa but I would like it to be inHg.

The MQTT yaml is:

  - name: "Node 1 Pressure"
    unique_id: "node_1_pressure"
    state_topic: "msh/US/2/json/LongFast/!67ea9400"
    state_class: measurement
    value_template: >-
      {% if value_json.from == 4038675309 and value_json.payload.barometric_pressure is defined %}
        {{ (value_json.payload.barometric_pressure | float) | round(2) }}
      {% else %}
        {{ this.state }}
      {% endif %}
    device_class: "pressure"
    unit_of_measurement: "hPa"

But when I switch it to convert the units you can see the graph will jump around from what I expect to 0, which it doesn’t do that if I don’t ask it to convert the values.

Graph when converted to inHg:

Graph when not converted:

It seems to be converting properly some times and not others. Just for confirmation I also used MQTTexplorer and got no random small values being reported:

I could do the conversion manually using a template sensor, but it feels like this functionality already exists in HA, but it is not working for me. Please let me know what I am doing wrong.