'vercel dev' command to test locally results in CORS/CSRF issues with releative and non-releative URLs to self.

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As the title implies, I am using the Vercel CLI and I am testing it locally through the 'vercel dev' command on a Svelte/Kit project. Everything works as expected, except POSTING data to self. I get a 403 Forbidden Error &
'Cross-site POST form submissions are forbidden' is returned. I fear this is to do with the origin, however I am unsure how to fix it.
I found a temporary solution which is to set the Content-Type header to 'application/json', instead of the default form data, although it would be nice for a fix to the real issue. Deploying through Vercel works great.