What is Adaptive battery as new feature of Pie OS?
With Android Pie(9.0) OS Google has rolled out a new features, Adaptive Battery.
Adaptive Battery works by learning your usage patterns for various apps and allows the apps you use regularly to stay in memory after you exit them.
Also apps you use rarely are killed as soon as you exit them so that they don’t run in the background and waste battery life or use Wi-Fi or mobile data.
Based on this algorithm, some applications that have been unexecuted during latest 3 or more days(user can change this period on following path, Settings > Device care > Battery > More > Settings > Put unused apps to sleep > Set sleep delay) and moreover have not been activated in the background, will be excluded from frequently used application list. Therefore you may not receive any notifications from those apps entered sleep and it is an intended operation by Google.