Device Tracker for mobile randomly not home (but I am)

I noticed recently that some of my automations aren’t working as there are conditions to when there’s someone home or not.

I have a template sensor that detects the state of my mobile and I can see the sensor switches off/on again in the history. Sometimes for a few seconds, sometimes minutes.
I set up a notification to let me know when it thinks I’m back home so I could track when it’s happening, but it seems random.

I’m not sure how this would happen as I’m pretty sure that if a phone is unavailable it just shows the last known state until HA gets an update. So in theory it’s thinking my phone is moving outside of my Home zone?

I have a couple devices, phone, watch, tablet, but only the phone is connected to my person and the only thing being tracked in the template sensor.

I was up and about early today, but haven’t been out.
“away” for 3mins then 1.5mins. Later in the day, just 10s

My sensor

- sensor:
    - name: abcdefg
      icon: >
        {% if is_state('device_tracker.my_phone', 'home') %}
        {% else %}
        {% endif %}
      state: >
        {% if is_state('device_tracker.my_phone', 'home') %}
        {% else %}
        {% endif %}

best to start with these steps then to look at the logs (as part of the same steps) to see what happened when the issue occurs