Last_update and last_changed incorrect

In the below alert - I I’m trying to alert my mobile if a chromecast has been left streaming - and also include the amount of time it has been streaming.

The alert itself works - after 60min - I get the notification - but the actual last_updated time given is incorrect, random minutes e.g. first 60min - the message said 7min - second 60min the message was 5min.

I suspect its because the actual media_player is updating meta-data (images,etc) as it plays - is there away of getting the time the actual state changed to playing?

    name: "Bedroom Chromecast has been left streaming for {{ relative_time(states.media_player.bedroom_tv.last_updated) }}."
    entity_id: media_player.bedroom_tv
    state: "playing"
    repeat: 60
    skip_first: true
    can_acknowledge: True
      tag: 1
      channel: Media
        - action: "StopBedroomStream"
          title: "Stop"
        - action: "PauseBedroomStream"
          title: "Pause"
      - cellphone_group

Yes, attribute changes will affect the last updated timestamp of an entity.

I guess you could trigger an automation when the stream starts playing and store a timestamp in an input_text to be used in your alert message.

Think I’m going to have to change the alert to a automation, I have quite a few devices, the above is just one - and don’t want to have tons of code for a repeatable notification.