Iis – WordPress wp-admin redirect loop behind IIS ARR Reverse Proxy

I've got a bit of a unique and interesting setup going on from what I can tell, with an interesting issue. I have a Windows Server 2012 box as my main web server hosting website.com. At http://website.com/sites/ I have a reverse proxy setup with ARR to point everything back to a WordPress WPMU install on a Debian box running Apache. Here's the rewrite rule configuration:

Now, everything works properly but the one thing messing up is that if I go to any wp-admin/ page through the reverse proxy, I get a redirect loop:

For the sake of being accurate with what I'm describing, here is my .htaccess file and my wp-config.php file (obvious stuff omitted).

What I find interesting is that if I change my local hosts file to point my domain to the Debian IP address and bypass the IIS ARR, then wp-admin/ works just fine, no issues. It just seems that my issue is somewhere with the translation between IIS and Apache? I'm at a loss for what to look at and debug this any further.

Any help would be super appreciated!