This flaw only happens in Ultimate mode, when the user explicitly tells the mux to switch to the discrete GPU. This is an extra feature only users who primarily use the laptop for gaming with an external display care about.
The laptop works fine in Optimus mode even with external displays, you just lost a bit of performance and you're missing out on some display features like G-Sync. So it is highly likely that most users always use the laptop in Optimus mode. If you primarily use the laptop as a laptop you probably wouldn't even know the mux feature existed.
The problem is Asus shipping extra features in their hardware that are not properly QA tested. It looks like they only thoroughly tested the golden path.