Q:

Are notifications lost forever during scheduled DnD in iOS12? 

In iOS12, notifications do not appear on are locked screen while scheduled do not disturb is in effect.  This is a new feature, and I can understand the reason it has been introduced. 
Is there any way to view the history of notifications that would have occurred during this time.  If one opens the notification screen (by swiping down), notifications that occurred before and after the do not disturb period are visible, but nothing in between. 
In previous versions, notifications could be used to view important news flash events that occurred overnight.  Is there a method to view these missing notifications or perhaps modify the new behaviour. 

Update:
I don't know why there were no overnight notifications the first time I used scheduled DnD after updating to iOS12 - maybe CNN had nothing to say for 8 hours.  But it seems to be working as documented now.  Both with Bedtime Mode enabled and without. 

A:

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 -> Do Not Disturb and be sure you have scheduled Do Not Disturb turned on.   Then switch on Bedtime Mode. 