Google resumes uploading and CORS

I am using google bootable drive API and in some cases see a lack of CORS headers. Before flying, OPTIONS always returns 200, but when making the actual requests, in some cases the headers are missing, which means I can't read the responses.

Not seeing CORS headers when: - loading final snippet - creating a resumable session - requesting session state

Viewing CORS headers when: - loading non-final snippet (answer to 308)