Oneplus incidentally maintains a whitelist of apps which are allowed to run in the background and only apps in that list can run for extended periods. While it is possible for users to add apps to that list, it comes with a set of apps already whitelisted ( like whatsapp). I found it out the hard way while working on Android.

You can read more about it here in case you are curious: https://hackernoon.com/notifications-in-android-are-horribly...