Windows Authentication in Microsoft Edge (Spartan) Does Not Prompt for Credentials

I have started to take a look at some of my apps on Microsoft Edge in the Windows 10 Preview build. The biggest issue I had up front, was that any of my internal sites that used Windows Authentication are not accessible (and these sites are recently developed applications that should be Spartan friendly from technology and layout perspective, which is why I want to utilize them in the latest browser).

For sites with Windows Authentication (which work fine on Chrome, Firefox, etc), I receive the following server error: 401 - Unauthorized: Access is denied due to invalid credentials.

This error is normal if I input the incorrect windows credentials in Chrome / Firefox / IE. However, on Edge I am never prompted for Windows credentials?

Is this functionality just not complete on Edge? Not planned for Edge? A bug? Or did I miss a setting somewhere that I overlooked on Edge?