Yeah, I played around with this a little bit. It seems like Notepad knows the document has been changed, the bug is purely that the title bar isn't updated in this one case.

Which actually seems all the more odd because it does get updated when you move the mouse, use the arrow keys (or other non-inserting keys like Alt+F4)

Oddly enough, I do. Seems to be a visual bug only.

Win 10 20H2 build 19042.928.