Like Gualtier Malde replied. The issue you are seeing with silent/background notifications is expected. And it has been going to that "muted" direction from IOS 11 onwards.. (or earlier).
https://developer.apple.com/documentation/usernotifications/setting_up_a_remote_notification_server/pushing_background_updates_to_your_app?language=objc
The system treats background notifications as low priority: you can use them to refresh your app’s content, but the system doesn’t guarantee their delivery. In addition, the system may throttle the delivery of background notifications if the total number becomes excessive. The number of background notifications allowed by the system depends on current conditions, but don’t try to send more than two or three per hour.
Topic:
App & System Services
SubTopic:
Notifications
Tags: