Redirect Firebase Hosting root to a Cloud Function is not working

I'm using Firebase Hosting with a firebase.json file that is supposed to forward all traffic to a cloud function (prerender) which populates meta and og tags for SEO.

{
  "hosting": {
      "public": "dist/prod",
      "rewrites": [
       {
          "source": "**",
          "function": "prerender"
       }
      ]
   }
}

My prerender function is is handling the request and rendering the HTML file. This works fine:

export const prerender = functions.https.onRequest((req, res) => {
   console.log('prerender function: "' + req.path + '"');
   ...
}

When hitting the end point at https://xxx.cloudfunctions.net/prerender, I correctly get the call in the Firebase Dashboard under Functions -> Logs:

prerender function: "null"

However, when calling https://mypage.firebaseapp.com, I get no logs and it seems to render the index.html inside of my dist/prod folder.

Is there anything I'm missing with the rewrites? I tried rewriting / to the same function, but no success. Any hints much appreciated!