The main camera is the one that should send the MQTT event, not the cameraai one. Just add it to the On Alert trigger list of the main camera (HD feed) and it should work as you expect. That's how I was running for a while until...
... I added MQTT support directly in my
alternate, Docker-based, approach. I really didn't like messing with
BlueIris for MQTT events and wanted it to come directly from the AI system to my Home Assistant install. That way I can have home automation control when to kick BlueIris into recording with advanced logic, e.g. when I'm on the deck enjoying the nice weather there's no need to trigger HD recording.