Ionic Local Notifications icon not shown when app is in background

Question

I need to customize the icon for local notifications, after reading few questions and answers:

How to change ionic 2 local push notification icon?

Ionic 2 : Local notification icon

I managed to make it work in the foreground, the icon is shown correctly.

The problem comes when the application is in background and recieves the notification, the notification is shown but the icon not, It shows a white square.

this.localNotifications.schedule({
      title: data.title,
      text: data.message,
      data: data,
      icon: 'ic_notification',
      smallIcon: 'ic_notification_small'
    });

I have two images in res/drawable both with transparent background, the big one is 192x192px and the small one is 24x24px, I don`t know what is smallIcon used for.