Fusion: AEM Assets Custom API module doesn't return Error Messages

We have a Fusion scenario which calls our AEMaaCS instance to make a Http Post call to a custom API (Sling Servlet). The success scenario runs as expected but when there is a validation error returned by the API, e.g., an HTTP 400 response, the module doesn't receive or process the error messages.
The API returns an error message in the body with the status code, but the AEM module in Fusion only processes the status code, not the message.