Template sensor not logging into History

I have a esp8266 with two MAX6675 sensors attached to it. I created a template sensor to calculate the Delta-T between the two sensors, and this seems to work just fine, but the dang thing will not record its history into, well, History, so I can’t reference in the past.
Here’s the code

sensor:
  - platform: wifi_signal
    name: "HVAC Plenum Temp WiFi Signal"
    update_interval: 60s
  - platform: uptime
    name: "HVAC Plenum Temp Uptime"
    update_interval: 60s
  - platform: max6675
    name: "HVAC Plenum Temp Supply Temperature"
    cs_pin: D7
    id: supply_temp
    update_interval: 10s
    device_class: "temperature"
    state_class: "measurement"
    accuracy_decimals: 1
    icon: "mdi:thermometer"
    unit_of_measurement: "°F"
    filters:
      - lambda: return x * (9.0/5.0) + 32.0;
  - platform: max6675
    name: "HVAC Plenum Temp Return Temperature"
    cs_pin: D8
    id: return_temp
    update_interval: 10s
    device_class: "temperature"
    state_class: "measurement"
    accuracy_decimals: 1
    icon: "mdi:thermometer"
    unit_of_measurement: "°F"
    filters:
      - lambda: return x * (9.0/5.0) + 32.0;
  - platform: template
    name: Delta-T
    id: delta_t
    update_interval: 5s
    device_class: "temperature"
    state_class: "measurement"
    accuracy_decimals: 1
    icon: "mdi:thermometer"
    unit_of_measurement: "°F"
    lambda: return (id(return_temp).state - id(supply_temp).state);

So if you’re wondering, one of the MAX6675’s is in the return plenum of the central air unit, the return being the side before the filter and the central air unit. The other is in the supply plenum, or the ductwork after the central air unit.
The purpose of this is to monitor the Delta-T between the return and supply air, which in a good working furnace should be between 16F and 22F when the HVAC is calling for cooling. I have an ecobee so I can see when then call for cooling happened.
I originally was going to do this as a helper outside of the esp8266 then use an automation to begin and end the recording of the Delta-T, based on the equipment_running state of my Ecobee thermostat, but didn’t really see any advantage to that. If the Delta-T is 5F or less, then it’s pretty safe to say that the unit is not calling for cooling, but if it’s like 8F, then there may be a problem. I plan to build an automation that checks for a Delta-T of >5F and <12F for more than 10 minutes to notify me on my phone.
Anyway, that’s the code and the backstory, so not sure what I am missing on why the template sensor is not logging it’s values into the history.

Have a plenty template sensors calculated by different formulas.
All have history in HA which fine visible.

IMHO need to wait some time after sensor implemented to see it history.

So I do see it in History when I click on that in the sidebar, but when I click on the entity I get “No statistics found”, which I guess is different than the state history???