Issue with Next.js preview API route not working on Vercel deployment

[08:23:05.343] Cloning <http://github.com/shadowbrand/shinecapital-next|github.com/shadowbrand/shinecapital-next> (Branch: develop, Commit: 006c3fd)
[08:23:05.913] Cloning completed: 570.535ms
[08:23:06.361] Installing build runtime...
[08:23:08.307] Build runtime installed: 1.946s
[08:23:08.939] Looking up build cache...
[08:23:09.413] Build Cache not found
[08:23:09.611] Installing dependencies...
[08:23:09.853] yarn install v1.22.17
[08:23:09.909] [1/4] Resolving packages...
[08:23:10.073] [2/4] Fetching packages...
[08:23:23.886] [3/4] Linking dependencies...
[08:23:23.888] warning " > next@12.0.7" has incorrect peer dependency "react@^17.0.2 || ^18.0.0-0".
[08:23:23.888] warning " > next@12.0.7" has incorrect peer dependency "react-dom@^17.0.2 || ^18.0.0-0".
[08:23:23.888] warning "next > @next/react-dev-overlay@12.0.7" has incorrect peer dependency "react@^17.0.2".
[08:23:23.889] warning "next > @next/react-dev-overlay@12.0.7" has incorrect peer dependency "react-dom@^17.0.2".
[08:23:23.889] warning "next > styled-jsx > @babel/plugin-syntax-jsx@7.14.5" has unmet peer dependency "@babel/core@^7.0.0-0".
[08:23:23.890] warning " > react-ga@3.3.0" has unmet peer dependency "prop-types@^15.6.0".
[08:23:23.892] warning " > styled-components@5.3.3" has unmet peer dependency "react-is@>= 16.8.0".
[08:23:30.691] [4/4] Building fresh packages...
[08:23:30.929] success Saved lockfile.
[08:23:30.940] Done in 21.09s.
[08:23:30.960] Detected Next.js version: 12.0.7
[08:23:30.962] Running "yarn run build"
[08:23:31.234] yarn run v1.22.17
[08:23:31.258] $ next build && next export
[08:23:31.906] Attention: Next.js now collects completely anonymous telemetry regarding usage.
[08:23:31.907] This information is used to shape Next.js' roadmap and prioritize features.
[08:23:31.907] You can learn more, including how to opt-out if you'd not like to participate in this anonymous program, by visiting the following URL:
[08:23:31.907] <https://nextjs.org/telemetry>
[08:23:31.907] 
[08:23:32.023] info  - Checking validity of types...
[08:23:32.151] warn  - No ESLint configuration detected. Run next lint to begin setup
[08:23:32.155] info  - Creating an optimized production build...
[08:23:32.176] info  - Disabled SWC as replacement for Babel because of custom Babel configuration ".babelrc" <https://nextjs.org/docs/messages/swc-disabled>
[08:23:32.721] info  - Using external babel configuration from /vercel/path0/.babelrc
[08:23:46.272] info  - Compiled successfully
[08:23:46.272] info  - Collecting page data...
[08:23:48.862] info  - Generating static pages (0/12)
[08:23:49.168] info  - Generating static pages (3/12)
[08:23:49.486] info  - Generating static pages (6/12)
[08:23:49.809] info  - Generating static pages (9/12)
[08:23:50.167] info  - Generating static pages (12/12)
[08:23:50.184] info  - Finalizing page optimization...
[08:23:50.189] 
[08:23:50.207] Page                                       Size     First Load JS
[08:23:50.207] ┌ ● /                                      2.54 kB         103 kB
[08:23:50.208] ├   /_app                                  0 B            87.3 kB
[08:23:50.208] ├ ○ /404                                   340 B          87.6 kB
[08:23:50.208] ├ ● /500 (304 ms)                          340 B          87.6 kB
[08:23:50.208] ├ ● /about                                 3.41 kB         114 kB
[08:23:50.208] ├ λ /api/exit-preview                      0 B            87.3 kB
[08:23:50.208] ├ λ /api/preview                           0 B            87.3 kB
[08:23:50.208] ├ ● /portfolio (325 ms)                    865 B           117 kB
[08:23:50.209] ├ ● /preview/[slug] (1873 ms)              2.04 kB         112 kB
[08:23:50.209] ├   ├ /preview/portfolio (357 ms)
[08:23:50.209] ├   ├ /preview/blue (347 ms)
[08:23:50.209] ├   ├ /preview/rainbow (310 ms)
[08:23:50.210] ├   ├ /preview/green (303 ms)
[08:23:50.210] ├   ├ /preview/about
[08:23:50.210] ├   └ /preview/team
[08:23:50.210] └ ● /team                                  3.46 kB         114 kB
[08:23:50.210] + First Load JS shared by all              87.3 kB
[08:23:50.211]   ├ chunks/framework-ddde1c8153ec7431.js   40.6 kB
[08:23:50.211]   ├ chunks/main-d36f011b027ca1e1.js        26.9 kB
[08:23:50.211]   ├ chunks/pages/_app-2a5cf2a1a7c7b089.js  18.2 kB
[08:23:50.211]   └ chunks/webpack-7e69b6628b5576da.js     1.72 kB
[08:23:50.211] 
[08:23:50.211] λ  (Server)  server-side renders at runtime (uses getInitialProps or getServerSideProps)
[08:23:50.211] ○  (Static)  automatically rendered as static HTML (uses no initial props)
[08:23:50.211] ●  (SSG)     automatically generated as static HTML + JSON (uses getStaticProps)
[08:23:50.212] 
[08:23:50.870] info  - using build directory: /vercel/path0/.next
[08:23:50.875] info  - Copying "static build" directory
[08:23:50.878] info  - No "exportPathMap" found in "/vercel/path0/next.config.js". Generating map from "./pages"
[08:23:50.879] info  - Launching 3 workers
[08:23:50.880] warn  - Statically exporting a Next.js application via `next export` disables API routes.
[08:23:50.880] This command is meant for static-only hosts, and is not necessary to make your application static.
[08:23:50.880] Pages in your application without server-side data dependencies will be automatically statically exported by `next build`, including pages powered by `getStaticProps`.
[08:23:50.880] Learn more: <https://nextjs.org/docs/messages/api-routes-static-export>
[08:23:50.880] info  - Exporting (0/6)
[08:23:50.880] info  - Copying "public" directory
[08:23:51.151] info  - Exporting (1/6)
[08:23:51.154] info  - Exporting (2/6)
[08:23:51.158] info  - Exporting (4/6)
[08:23:51.200] info  - Exporting (6/6)
[08:23:51.209] Export successful. Files written to /vercel/path0/out
[08:23:51.213] Done in 19.98s.
[08:23:52.857] Generated build outputs:
[08:23:52.858]  - Static files: 73
[08:23:52.858]  - Serverless Functions: 0
[08:23:52.858]  - Edge Functions: 0
[08:23:52.858] Deployed outputs in 1s
[08:23:55.605] Build completed. Populating build cache...
[08:24:04.204] Uploading build cache [39.56 MB]...
[08:24:06.050] Build cache uploaded: 1.846s
[08:24:06.073] Done with "package.json"

// ./web/pages/api/preview.js

export default function preview(req, res) {
  console.log(req, res)
  if (!req?.query?.secret) {
    return res.status(401).json({message: 'No secret token'})
  }

  // Check the secret and next parameters
  // This secret should only be known to this API route and the CMS
  if (req.query.secret !== process.env.SANITY_PREVIEW_SECRET) {
    return res.status(401).json({message: 'Invalid secret token'})
  }

  if (!req.query.slug) {
    return res.status(401).json({message: 'No slug'})
  }

  // Enable Preview Mode by setting the cookies
  res.setPreviewData({})

  // Redirect to the path from the fetched post
  // We don't redirect to req.query.slug as that might lead to open redirect vulnerabilities
  res.writeHead(307, {Location: `/preview/${req?.query?.slug}` ?? `/`})

  return res.end()
}