We had a similar issue. But the corrupt was the MCID and our Visitor ID were not being stitched since our visitor ID was not populating in our digital data layer when the MCID service ran. We use DTM for implementing our tools (analytics, target, and MCID). Our website is in angular so the data layer didn't populate before MCID service ran since it runs in an iframe on it's own without needing to wait for other analytics. Once we fixed that issue, our trigger event in ACS fired and the email was sent.
It was hard to determine since our trigger was working and I could see the MCID sent in an Evar but without the syncing of the ID's working the event failed in ACS since the integration is reliant on the customer attributes and authentication of MCID stitched to Customer ID.