Disable notification spam at end of do not disturb schedule
It seems like an Oreo issue as documented here:

https://www.androidpolice.com/2017/09/26/feature-bug-oreo-dnd-turns-off-get-barraged-sounds-vibrations-pending-received-notifications/

It was resolved in 8.1.0, but Samsung hasn't provided this update to its 8.0 phones. So today it is mostly Samsung users that are affected.

https://us.community.samsung.com/t5/Galaxy-S9-Questions-and-Answers/S9-notifications-after-do-not-disturb-goes-off/m-p/371390#M7828