I am guessing you are referring to @ing somebody
rather than an actual Flag
If so, by default I think you should get an INTERNAL notification like this in your profile menu
which you can also filter
But if you have
Notifications turned off, you won’t get EXTERNAL notifications
Do not disturb turned on
or
Blocked user
If you want to get Notifications, when you switch it on you should get an acknowledgement notification like this
Due to sporadic activity and multiple timezone users, I would reccomend, if you want the Notifications, you can actually set a notification schedule