Firebase function not returning edited index HTML content

I have a Vue app deployed on Firebase Hosting. Until recently we had no need to add dynamic Open Graph tags to it but now we do, and the path we took is a Firebase Function that executes for all the paths when the app should be served.

{
  "hosting": {
    "public": "dist/spa",
    "ignore": [
      "firebase.json",
      "**/.*",
      "**/node_modules/**"
    ],
    "rewrites": [
      {
        "source": "**",
        "function": "host"
      }
    ]
  },
  "functions": {
    "source": "functions"
  }
}

You can notice in the firebase.json file above that there is a function called host that is executed for all paths. And this is working fine, I can see the execution of the function in the console, I also checked the logs and it is modifying the index.html file as expected, but at the end it looks like the original index.html (which is found under dist/spa) file is returned to the user. The code is bellow:

const functions = require('firebase-functions');
const fs = require('fs');

exports.host = functions.https.onRequest((req, res) => {
  let indexHTML = fs.readFileSync('./hosting/index.html').toString();

  console.log('original: ', indexHTML);

  // const userAgent = req.headers['user-agent'].toLowerCase();
  const ogPlaceholder = '<meta name=functions-insert-dynamic-og>';

  // Cache the response for 10 minutes
  res.set('Cache-Control', 'public, max-age=600, s-maxage=1200');
  indexHTML = indexHTML.replace(ogPlaceholder, getOpenGraph());

  console.log('modified: ', indexHTML);

  res.status(200).send(indexHTML);
});

const desc =
  'Desc here';
const imageURL =
  'imageUrlHere';
const title = 'Title Here';

const getOpenGraph = () => {
  return `<meta property="og:title" content="${title}">
          <meta property="og:site_name" content="SiteName">
          <meta property="og:url" content="URL">
          <meta property="og:description" content="${desc}">
          <meta property="og:type" content="website">
          <meta property="og:image" content="${imageURL}">`;
};

During our build process the Vue template index file is copied over to function/hosting. That file is read using fs and a placeholder string is replaced with the appropriate Open Graph tags. But the source that is served is still the original one...