应用程序在后台时，UILocalNotification自定义声音的持续时间为5秒

I am using deprecated UILocal notifications, and use custom tracks of .caf type, the duration of the audio is 27 seconds. When device is locked and notifications arrives so the sound plays fully, but when the app is in the background mode and device is not locked, the notification appears and sound plays but just 5 seconds. Could you please advice is there any documented Apple suggestions, or why it is like that? I am using deprecated UILocal notifications, an