Updating tickets on behalf of unverified emails

I'm using the Request API to submit new comments on a ticket on behalf of the original ticket creator. Tickets are automatically generated through a checkout process requiring payment with an account so from our perspective the emails are verified.

We are using the OAuth process to generate a token for an admin user with the impersonate scope.

Data being sent is as below:

https://cleansed.zendesk.com/api/v2/requests/{ticket Id}
-d {"request":{"comment":{"type":"Comment","body":"Testing, please ignore"}}}
-h "X-On-Behalf-Of: "{Zendesk User ID}"
-h "Bearer" : "{OAuth Token}"

The response we are getting is 

{ "error": { "title": "Forbidden", "message": "Unverified email address" } }

I even tried using the User API to set the verified flag to true, then wait 10 seconds before running the Requests API process.

The same request to a ticket with a verified email in Zendesk does work (although it sends the email to the ticket creator, not the ticket steward).