Are notifications lost forever during scheduled DnD in iOS12?
Source:

If you set a scheduled Do Not Disturb time (as many of us do during our normal sleeping hours), you’ll have the option to toggle Bedtime Mode for those hours. 

Regular Do Not Disturb mode silences calls and notifications. When it’s active, sounds and vibrations won’t wake you up (or disturb your dinner companion, or make noise in the movie theater), but they’ll still appear on the lock screen.

Bedtime Mode adds two new behaviors to Do Not Disturb. 


It dims the display considerably, so if you pick up your phone in a dark room—to check the time, for example—it won’t blind you with bright light.
It sends incoming notifications straight to the notification history, skipping the lock screen entirely.


To enable Bedtime Mode, tap on Settings -&gt; Do Not Disturb and be sure you have scheduled Do Not Disturb turned on.  Then switch on Bedtime Mode.