PushNotification on iOS not working in background

Description:

Hi everyone,
I have an issue with the whitelabel version of RocketChat.
I followed the whitelabel documentation to customize my app, and set the push notification secs as explained: APN development and production key/certificate/psw, GCM API Key/Project number.

The problem is that, while the Android push notifications work just fine, when sending to iOS the Push notification service only works when the app is not in background.

Any suggestions?

Steps to reproduce:

1. Set up push notification as explained on https://docs.rocket.chat/guides/developer/mobile-apps
2. Set up APNKey, APNCert for production and development
3. Send message to an account

Expected behavior:

When sending messages to the user, a push application should be displayed when the application is running both in foreground and background, on ios and android.

Actual behavior:

When sending messages to the user, a push application appears on android, whit the application in background and foreground, while on ios it appears only when the application is running foreground.

Server Setup Information:

• Version of Rocket.Chat Server: 3.8.0
• Operating System: Linux
• Number of Running Instances: 1
• Oplog: enable
• NodeJS Version: v12.18.4
• MongoDB Version: 4.0.21

Client Setup Information

• IPhone 11 Pro:
• Operating System: iOS 14.2

Start with the mobile-app setup guide linked in the issue and reproduce the notification behavior on iOS 14.2 with the app backgrounded. The issue names no source file or test; the work is complete when configured iOS push notifications appear reliably in both foreground and background.