Hi Stephani,
Is Event Attendance a custom entity?
Source Customer Journey is an OOB field of contact entity, you can find it in Details > MARKETING, the field will be only populated if contact record is created from a customer journey.
If your contacts had existed in crm, then the field will never be populated, so that would be the reason that why your field remains blank.
Currently workflow in customer journey couldn't detect customer journey that included itself, that because cj workflow run on Contact entity,
while the only relationship between contact and customer journey is N:1, the source customer journey field of contact.(It's in Contact > N:1 relationship)
Please set your field to a specific customer journey value.(which means you should create multiple workflows if you would like to create event attendance from different customer journeys)
Regards,
Clofly
