The "Viewing Notifications" section of the Work Center guide has a note about privileges and Administrative settings.

Thanks for your hint, but that note is not applied to my case. My user has full privileges but his Notification tab always show 0 notification even though he received some notifications via email.

Thu, I see that you are logged in as someone else.  We had an issue with loginAs at some point.  If you are logged in as yourself or viewing another's browser, do you see the correct notifications?

This article explains the "View Notifications".  ITOM Practitioner Portal (microfocus.com)

If I logged in as other users, I was still able to see tons of notifications from that Notification tab, but not with this specific user; that's why I really don't know what happened to this user account even though he has "View Notifications" privileges for all items and he received notification emails as well.