I remember reading about this on the old Ubuntu MATE community Discourse and never really thought about as it never affected me all these years until this morning!
I always update/upgrade through terminal and I do this every morning and today was no different. After a small update for whatever reason the gui updater popped up saying I needed a restart and there it was the UN flag in my upper panel! A restart made it go away and then I remembered the thread from the old UM Discourse so I revisited that thread for the solution.