Hi ldavies,
That because workflow tile in customer journey is only based on contact or account records.
https://docs.microsoft.com/en-us/dynamics365/customer-engagement/marketing/customer-journey-tiles-reference#launch-workflow
If you want to create an auto event registration record for members in customer journey,
just create an on-demand workflow, still set the source entity to contact and add a step to create an event registration record:

in properties setting, add dynamic contact value to the event registration form fields.

Then build a customer journey like below:(a trigger tile to detect specific link in your marketing email clicked, and you might add your custom tile in customer journey, so be sure to add the trigger tile after marketing email tile)
The workflow will automatically track contacts in the customer journey (recipents who clicked link)

Result in CRM:

Regards,
Clofly