Automation MQTT trigger not firing

I have an automation I want to trigger from a camera/motion_image topic. I can see in MQTT explorer the topic is being published to.

The topic is also used by an MQTT Camera entity and is working as expected there.
However my automation is never triggered (confirmed by trace) by this MQTT trigger. I have tried using a different topic (camera/motion) as a trigger and the automation runs correctly.
Is it possible the size of the payload (a ~600kb jpeg) is interfering with the trigger operation?

alias: Drive motion notification
description: ''
trigger:
  - platform: mqtt
    topic: camera/motion_image
    id: image
condition: []
action:
  - service: camera.snapshot
    data:
      filename: /extsd/ipwebcam/snapshot.jpg
    target:
      entity_id: camera.drive_motion_detection_image
  - device_id: ad91f0fdf7dae795ff215c2440663373
    domain: mobile_app
    type: notify
    message: |
      {{ trigger.id }}
    data: >
      {% set android_data = {"image": "%s"} |
      format("/media/camera/snapshot.jpg?authSig=REDACTED")
      %} {{ android_data }}
    title: Drive camera
mode: single

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