Ding! Ding! D-D-D-D-D-Ding! Ding! Ding! Ding! D-D-D-Ding! Ding!
That's the sound you will hear every morning if you're on Android Oreo and you use Do Not Disturb during the night. For some reason, DND and notifications work differently since the latest update to Oreo: when your phone exits DND, it will proceed to play the sound or do the vibration of every notification that was received during the time it was stuck in DND.
We're not sure if this is a bug or an intended behavior, though the latter doesn't make a lot of sense. Previously, when your phone exited DND, whether at a set time or on an alarm, it was happy to just display the notifications so you'd see them when you picked it up. It made sense because who wants to deal with a barrage of sounds the moment they open their eyes? Now, when DND is off, all those notifications that were already received (this doesn't have anything to do with Doze stopping notifications from getting to you) and silenced because DND was on, queue up their sounds in a melody of dings and rings and vibrations that would wake the dead up if you're popular enough to get several notifications at night.
That's the case of Artem (#artemsluck) who first noticed the issue because he's so popular he gets hundreds of notifications when he's asleep, and he usually sets DND to disable a short while before his alarm (he wants to be reachable just in case), so now he's getting bombarded with sounds before his alarm triggers. Note that this is a new behavior that came with the latest Oreo update: Artem has been doing the same thing for a while and the notifications never queued up their sounds before.
Anyway, while we do understand part of the logic behind this: the phone wants to get your attention because you received notifications while you were under DND and you haven't seen them yet, this could easily be achieved with one or two rings. No need for the whole orchestra. The "solution" for now, as suggested by users, is to wake up before DND turns off (ie, no way to use "DND until next alarm"), so you have time to deal with those received notifications and avoid hearing their cacophony of sounds.
The issue was reported by Artem on Google+ and Twitter, where several users chimed in with their frustration and woes, as well as their suggestions, investigations, and sometimes criticism for the way Artem sets things — he's just weird you guys, and there's #artemsluck, never forget that!
But we later discovered that there was another issue with notifications on Oreo: some people are receiving double notifications too, that plus the notification melody on exiting DND (comment 1 and 2). Fun stuff. A quick look through the Issue Tracker turned up a couple of potentially related bug reports, but it doesn't seem like this has been effectively documented and reported there.
Let us know if you're facing the same notification bombardment situation, and if so what you've done to alleviate it. The comment section is also open for all bit**ing and moaning about Google's knack for "fixing" things the wrong way lately.