OnePlus - Oxygen OS - No Notifications - App Hibernation

1.)

System Settings -> Battery -> Menu -> Aggressive Doze & App Hibernation

--> Turn these off completely or at least select "COL Reminder"

2.) System Settings -> Battery -> Menu ->  High Power Usage apps

If COL Reminder is shown here, disabled them.

3.)

System Settings -> Advanced -> Recent App Management -> Normal Clear should be set, not Deep Clear

 

4.)

Apps -> Settings -> Apps Auto Launch

--> Turn this on in the top right corner and turn it on for COL Reminder