Doze and App Standby has been added to help reduce battery consumption.
From app settings, you can whitelist apps to prevent it from being affected by Doze or App standby.
Some apps are already exempted such as critical system apps.
If you are not receiving notifications from a particular app, like a third party alarm clock, you can add the application to a whitelist that will ignore the battery optimizations.
As a workaround to ensure notifications are received
1. Go to Settings> General tab (if applicable)> Battery & power saving> Battery usage> Ignore optimizations.
2. Tap the app to ignore battery optimizations.
Ignoring battery optimizations for a particular app may adversely affect battery life.
We recommend the consumer notify a particular app developer if they experience this issue using their app so they can update the app to take advantage of the battery optimization.