Email Body Empty When Sending DocuSign Email via Apex Using Messaging.renderStoredEmailTemplate

Hi, I am sending a DocuSign email from Apex and using the Salesforce Email Template to generate the Email Body. I am using Messaging.renderStoredEmailTemplate to get the HTML body, and when I check the result using System.debug the HTML content is correctly generated and present.

However, after the email is sent, the email body is completely blank.

Relevant Code:

Messaging.SingleEmailMessage renderResult = Messaging.renderStoredEmailTemplate(
       opportunityEmailTemplateId, opportunityId_i, myContact.Id
);

String opportunityOfferEmailContent = renderResult.getHtmlBody();

myEnvelope = myEnvelope.withEmail(
       opportunityOfferEmailSubject, //Subject
       opportunityOfferEmailContent //Email Body
);

myEnvelope = dfsle.EnvelopeService.sendEnvelope(
       myEnvelope,
       true
);

How do I resolve this?

Kind Regards

Willy