Oracle digital assistant: CORS error on request done by externally hosted webview.

Summary:

I have developed an externally hosted webview following the documentation here Webviews (oracle.com), it consists of a nodejs, express intermediary service that builds an url with the URL of an Angular SPA and the skill's callback URL as a query parameter. This intermediary service works perfectly, and ODA is able to perform the GET request to load my SPA. However, when trying to make the POST request from my SPA to ODA using the callback URL, I encounter a CORS error

As I understand, the nature of this error must be dealt with on server side, ODA in this case. I am admittedly confused because I have no issue whatsoever when communicating back and forth between ODA and the intermediary service, however my SPA is unable to make the POST request to the callback URL that ODA needs to take back control.