That is a bug in a security fix.
The fix is that the code of the configuration page no longer contains your Username and Password in clear texst, see:
http://dnlongen.blogspot.nl/2014/04/CVE-2014-2719-Asus-RT-Password-Disclosure.html
The bug is that the strenght checker and show password function are somewhat broken because they can no longer find your Username and Password in the page code.
The checker does work at the moment you type in a new password.
BTW, I did a quick check in the GPL source code of another major router manufacturer, and I found the same problem: they also are embedding the router password right into the HTML code...
One can only wonder at the people who write those router firmwares. Backdoors, cleartext passwords in HTML page (meaning a malicious website could potentially retrieve that info if you have a valid session open on your router in another tab). UPnP answering to WAN side requests...