Opera team please pay attention to web notifications, which unfortunately do not work as desired. Web notification is very convenient, an advanced feature, I do not have enough of it to get the maximum pleasure from using the Opera browser.
While I'm not sure about every past instance of the problem on my end, since a few days ago I noticed that more or less every day the problem showed up. I thought "Wth is going on ? What possible change did I introduce in my OS/app usage that may have escalated this ?". Then it hit me that I have been away from home often while PC is on by switching user (not logging off) so that no one can access and mess up my account / current session.
So it seems after "logging in" back to my account / ongoing session, Opera runs into the problem that is the subject of this thread.