Hey @Arsenalist thanks for the update.
I don’t think that information is correct either when it comes to the /userinfo audience, see this post.
While I’m still not positive what the issue could be in your particular implementation, I spent some time digging into this today and was successfully able to call fetchUserInfo() - I added some code to our auth0-express-web-app sample to accomplish this. If you are interested in trying my updated sample yourself, you can find it here.
You should just need to clone it, cd into 01-login, npm install, add your own .env file (below), and run npm start:
CLIENT_ID={your_client_id}
ISSUER_BASE_URL={https://your_domain.us.auth0.com}
SECRET={random_string}
clientSecret={your_client_secret}
PORT=3000
Hope this helps!