Set bedtime notification to 5 minutes before and then set an alarm to your duration target time + 8 minutes. Then please wait for 3 minutes and if the notification will not show up, please send us the application log by using Left ☰ menu → Support → Report a bug.
This is a companion discussion topic for the original entry at https://sleep.urbandroid.org/docs/faqs/bedtime_notifications_missing.html