Candidate's preferred communication channel

Summary:

"The candidate can't receive any emails because email isn't set as their preferred communication channel."

We are testing an integration and have noticed that when creating the candidate via API we see this warning message. We are using HDL to create the application. A subsequent HDL load seems to be needed to clear the flag (EMAIL_PREFERRED_FLAG). The requirement is to load candidates and progress them to Offer, To be created. In testing we have found that the offer letter does in fact go out to the candidate. However, I've noticed that attempting to send the candidate an email from the Messages tab, the compose button is grayed out.