Thingworx Asset Advisor custom alert-notification handler

I'm trying to set up a custom alert notification handler in Asset Advisor, Thingworx 8.3. I have the handler defined & set up as per the customization guide, but I cannot figure out how to get the service called. When I execute the GetNotifcationHandlers service from composer, it works as I would expect - it returns the handlerID, serviceName, etc.

I also have the handler set up in the launchpoint configuration table in the AdditionalAlertNotificationHandler field.

When I manually cause an alert to be fired, the email notification works, but my custom handler is not called. I have a feeling there is some other config that needs to be done somewhere, but I really cant find where that is. No evidence n log files that my handler is even attempted to be called - no permission warnings, etc.

Any ideas?