Hi @DCrinnion,
This is hard to say from a distance, without being able to poke around your portal. My hypothesis would be that the contact was on a suppression list at the time of the first attempted enrollment and now they’re not anymore. If the suppression lists are filtering based on email or email domain and if the email address was since updated, that would also explain why.
Generally I find exclusion and inclusion in HubSpot workflows very reliable - but of course, contacts will enter and leave active lists as they meet or do not meet list criteria anymore. That’s probably what’s going on here.
I’m not aware of a way to see list membership of a contact at specific points in the past. If you want an official confirmation on my above hypothesis, you would have to either investigate deeper into the list criteria and changes on the contact record yourself or reach out to HubSpot support.
Let me know if you have any follow-up questions!