Since Android 12 (API 31) notifications produced by startForeground method wait 10 seconds until showing. This could be confusing to user who use DownloadNotificationHelper methods to build notifications to show in a custom DownloadService (protected abstract Notification getForegroundNotification(List<Download> downloads, @RequirementFlags int notMetRequirements);).

Docs excerpt:

Devices that run Android 12 (API level 31) or higher provide a streamlined experience for short-running foreground services. On these devices, the system waits 10 seconds before showing the notification associated with a foreground service. There are a few exceptions; several types of services always display a notification immediately.

To make the notifications be shown immediately once actual download operation is started, FOREGROUND_SERVICE_IMMEDIATE can be set to NotificationCompat.Builder.setForegroundServiceBehavior().
See: https://developer.android.com/guide/components/foreground-services#notification-immediate