[Athari]

Maximized windows have non-rounded corners. This has been the case for all themes designed by Microsoft since Windows XP when rounded corners were first implemented.

Some apps may override system chrome with pretty rounded nonsense and not bother with implementing non-rounded maximized state, or mess with proper maximization of windows, or you may install bad non-Microsoft skins. These are app-specific problems though, Microsoft can't do anything about it.