Hi @sbarty,
I heard back from our developers.
According to them, it is expected behavior that if a user is canceled or denied from a webinar, he cannot be added back.
If you want to add the same attendee back to the webinar, consider calling the update a webinar registrants status, and set the action to approve.
Here is how a sample request would look like:
PUT /webinars/{webinarId}/registrants/status?occurrence_ids=1,2,3 body: { "action": "approve", "registrants": [ { "id": "123abc", "email": "user@example.com" } ] }