I have a samsung Galaxy S7 with Android 7.0 and my calendar notifications for events won't go off unless I have the calendar app open in the background. I already added the calendar app to the list of apps I never want to go to sleep in Settings - Device Maintenance - Battery - Unmonitored apps. Also Power Save mode is always off. I have researched this issue but it seems like for those who are affected, there is no fix. I have tried downloading other calendar apps and synced them with my Google calendar and they will notify for a few events and then stop. And I add them to list of Unmonitored app too. If anyone has found a fix that had this issue I would love to know it. Thanks!