AuthJS doesn't work with vercel dev using SvelteKit

[nicodes]

Hello! I have a new SvelteKit project that I've added AuthJs to using the Google provider. This works both when deployed to Vercel, and when run locally with pnpm dev. However this breaks when running locally with vercel dev. Specifically after hitting the /signin route with the error:

Request URL: http://localhost:3000/signin
Request Method: POST
Status Code: 403 Forbidden
Remote Address: [::1]:3000
Referrer Policy: strict-origin-when-cross-origin

And this message in the response: Cross-site POST form submissions are forbidden

To reiterate, AuthJS does still work when running pnpm dev and only gives this error with vercel dev. I think it's likely that others may have run into the same issue because vercel dev is needed to run the project using Vercel storage. I read through the docs for both AuthJS and Vercel CLI but haven't been able to find what's causing the issue, any help would be greatly appreciated!