MQTT Trigger Sensor temporarily gets value "null" when reloading template sensors

Configuration
1

I have defined an mqtt platform trigger sensor like this:

  - trigger:
      - platform: mqtt
        topic: house/watermeter
    sensor:
      - name: "Water Meter"
        state: "{{trigger.payload}}"
        unit_of_measurement: "m³"
        state_class: total_increasing
        device_class: water
      - name: "Water Meter Latest Reading"
        state: "{{as_timestamp(now())|timestamp_custom('%Y-%m-%dT%H:%M:%S')}}"

I have a mobile app where I enter a value manually and post it on the mqtt topic, hence the construction.

The sensors above stores the water meter value in one sensor and the timestamp of the reading in the other. Note that the latest reading is just the timestamp of when the topic is read by home assistant. However it works. My latest reading was 3 days ago and the timestamp also reflects that. When I reload my configuration, the timestamp is also preserved like i want. However, there is a short time span during the reloading that the latest-reading-sensor reports "null" and so any automation based on that time sensor triggers.

How can it be that the timestamp sensor report null?

2

Show those triggers.

3

Example of automation that triggers:

alias: Laundryroom
description: ""
triggers:
  - trigger: state
    entity_id:
      - sensor.water_meter_latest_reading
actions:
  - action: input_select.select_option
    metadata: {}
    target:
      entity_id: input_select.daniels_location
    data:
      option: Laundryroom
mode: single
4

Trace for automation going from valid state to null:

this:
  entity_id: automation.laundryroom
  state: 'on'
  attributes:
    id: '1778965916131'
    last_triggered: '2026-05-19T21:20:22.121102+00:00'
    mode: single
    current: 0
    friendly_name: Laundryroom
  last_changed: '2026-05-19T21:25:33.839260+00:00'
  last_reported: '2026-05-19T21:25:33.839260+00:00'
  last_updated: '2026-05-19T21:25:33.839260+00:00'
  context:
    id: 01KS122E0FAS2C80ER89HMFVWY
    parent_id: null
    user_id: null
trigger:
  id: '0'
  idx: '0'
  alias: null
  platform: state
  entity_id: sensor.water_meter_latest_reading
  from_state:
    entity_id: sensor.water_meter_latest_reading
    state: '2026-05-16T22:42:07'
    attributes:
      friendly_name: Water Meter Latest Reading
    last_changed: '2026-05-19T21:20:22.096329+00:00'
    last_reported: '2026-05-19T21:20:22.096329+00:00'
    last_updated: '2026-05-19T21:20:22.096329+00:00'
    context:
      id: 01KS11RXJG02XC58G8CYE5MW4G
      parent_id: null
      user_id: null
  to_state: null
  for: null
  attribute: null
  description: state of sensor.water_meter_latest_reading
5

And the trace a tiny bit later showing state going from null to its previous value:

this:
  entity_id: automation.laundryroom
  state: 'on'
  attributes:
    id: '1778965916131'
    last_triggered: '2026-05-19T21:20:22.121102+00:00'
    mode: single
    current: 0
    friendly_name: Laundryroom
  last_changed: '2026-05-19T21:25:33.839260+00:00'
  last_reported: '2026-05-19T21:25:33.839260+00:00'
  last_updated: '2026-05-19T21:25:33.839260+00:00'
  context:
    id: 01KS122E0FAS2C80ER89HMFVWY
    parent_id: null
    user_id: null
trigger:
  id: '0'
  idx: '0'
  alias: null
  platform: state
  entity_id: sensor.water_meter_latest_reading
  from_state: null
  to_state:
    entity_id: sensor.water_meter_latest_reading
    state: '2026-05-16T22:42:07'
    attributes:
      friendly_name: Water Meter Latest Reading
    last_changed: '2026-05-19T21:25:50.172552+00:00'
    last_reported: '2026-05-19T21:25:50.172552+00:00'
    last_updated: '2026-05-19T21:25:50.172552+00:00'
    context:
      id: 01KS122XYW02QYJFZZBV8PG2Z9
      parent_id: null
      user_id: null
  for: null
  attribute: null
  description: state of sensor.water_meter_latest_reading
6

Try:

triggers:
  - trigger: state
    entity_id:
      - sensor.water_meter_latest_reading
    not_to:
      - unknown
      - unavailable