AP Invoices Callback and Event monitoring

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I have a successful OIC Integration which uploads AP Invoice FBDI zip file and associated parameters file to ERP Cloud.  I also have a registered event subscription created via another ERP Callback integration.  I have checked the subscriptions using:

     http://server:port/soa-infra/PublicEvent/subscriptions/

I cannot get the Callback to work as expected.  I wait for the Import Payables Invoices job to complete and expect the callback event to trigger, but it does not.  HOWEVER, once it did fire 17 minutes after the first 'FBDI' integration but it errored with the following:

Primary Identifier: undefined

Q1. How can we check whether an Event has fired