Next JS Rewrites for Sanity CMS giving 404 page after login

I'm currently hosting a Next JS site on Netlify with a Sanity CMS backend for content editing. I've got it configured to run on the single domain IE - examplesite.com has the front-end website, where examplesite.com/sanity opens the Sanity Studio editor.

This works as intended if the editor has a log-in cookie already (from previous login, etc), but if the editor has to login, after going through the login process the site is redirected to /sanity/desk and Next displays a 404 page. The editor then has to visit /sanity to access the CMS, where it redirects successfully to /sanity/desk (no 404).

Following the official Sanity setup guide, I have the following rewrite applied in the Next config.

const SANITY_REWRITE = {
  source: "/sanity/:path*",
  destination:
    process.env.NODE_ENV === "development"
      ? "http://localhost:3333/sanity/:path*"
      : "/sanity/index.html",
};

const DESK_REWRITE = {
  source: "/sanity/desk",
  destination: "/sanity/index.html",
};

const withBundleAnalyzer = require("@next/bundle-analyzer")({
  enabled: process.env.ANALYZE === "true",
});

module.exports = withBundleAnalyzer({
  reactStrictMode: true,
  async rewrites() {
    return {
      beforeFiles: [SANITY_REWRITE, DESK_REWRITE],
    };
  },
  images: {
    domains: ["cdn.sanity.io"],
  },
});

For completeness, here is my sanity.json as well:

{
  "root": true,
  "project": {
    "name": "main-site",
    "basePath": "/sanity"
  },
  "api": {
    "projectId": "ga8f69l8",
    "dataset": "production"
  },
  "plugins": [
    "@sanity/base",
    "@sanity/components",
    "@sanity/default-layout",
    "@sanity/default-login",
    "@sanity/desk-tool",
    "@sanity/dashboard",
    "dashboard-widget-netlify"
  ],
  "env": {
    "development": {
      "plugins": ["@sanity/vision"]
    }
  },
  "parts": [
    {
      "name": "part:@sanity/base/schema",
      "path": "./schemas/schema"
    },
    {
      "name": "part:@sanity/desk-tool/structure",
      "path": "./structures/deskStructure.js"
    },
    {
      "implements": "part:@sanity/dashboard/config",
      "path": "src/dashboardConfig.js"
    }
  ]
}

My assumption was that any path after /sanity would be redirected to the sanity/index.html file output by the build command (build command below), but it doesn't seem to be functioning correctly for any path except just /sanity.

Build command (in package.json):

{
  "scripts": {
    ...
    "prebuild": "echo 'Building Sanity to public/sanity' && cd sanity && yarn && npx @sanity/cli build ../public/sanity -y && echo 'Done'",
   ...
  },

Any help greatly appreciated!