All of a sudden, my notifications for any given app stopped being displayed. Like most, I searched the Internet for solutions, but unfortunately, I could not find any that would help my situation.
My very cumbersome solution was to reset all the apps' settings, which required adding permissions again one by one. Still trying to figure out what the cause was in the first place, I backtracked which and how I am using my apps. Lucky me, I found the culprit.
I use WhatsApp Business with a free Bot App called WaBot to automate my responses 24/7. Whenever I had WaBot activated (permitting access to notifications necessary to work with WhatsApp), the general notification settings seemed to undo themselves, although all settings for all Apps were active.
Once I deactivated the WaBot settings, my notifications returned functional! Go figure. Long story short, one of the reasons your notifications stopped working might be due to another App.
