Emails showing HTML code when being sent via API

When I try to email an invoice to a customer using the Zoho Invoice API, the customer sees a bunch of html code beneath the subject line (in the message preview). Please refer to the following screenshot taken from my iPhone's native Mail application:

The top email was sent using the Zoho Invoice web app on desktop. The bottom email was sent using the Zoho Invoice API, via a custom function within Zoho Creator. They are the exact same email with the exact same body. When I send it via your web app, the preview text shows my desired result. But when I send it via API, the preview text instead shows the HTML code used to build the email. This makes our emails look like SPAM to our customers, which is terrible.

After much trial and error, it seems to me that if I do NOT include the "cc_mail_ids" parameter, the issue is fixed. Therefore, I'm assuming there's a problem with Zoho's API. Please have your developers look into this. We shouldn't have to choose between CC'ing someone and making our emails look like spam.

Thank you!

Josh