Web Alert on One Plus phones

One Plus phones use an operating system called Oxygen, which includes a background agent that kills apps that perform many background calculations. Users that let Web Alert perform frequent checks often open the app and see that no checks have been made for hours.

This has a solution:

1. Press the square button at the bottom of the screen, which will show all recently opened apps.
2. At the top right of each app you can see a lock sign. Press it to close the lock. The app will now be pinned/locked in the recents screen and its background process will not be killed anymore.