Why is Firebase's Realtime Database's child_removed event handler not executing?

My database contains:

user: {
  'monkey': 'banana',
  'bear': 'fish'
}

Why does the event handler not execute?

let db = firebase.database();

let user = db.ref('user');

user.on('child_removed', (e) => {
  console.log(e.val());
});

user.child('monkey').remove();  

from Why is Firebase's Realtime Database's child_removed event handler not executing?