iis - URL Rewrite Not Firing Properly

I had an url rewrite rule configured at IIS Root (ApplicationHost.config) that redirects www.example.com to HTTPS. The URL Rewrite configuration is setup on an IIS Instance behind a Public Load Balancer of our hosting where our SSL is installed by our hosting provider. The Load Balancer provides us with X-Forwarded-Proto header that sets to http if the original request is in HTTP and https if the original request is in HTTPS.

The Url Rewrite is configured this way:

 Pattern: .*
 Conditions: Match All
             Input                   |Type                |Pattern
             -------------------------------------------------------------
             {HTTP_HOST}             |Matches the Pattern |www.example.com
             {HTTP_X_Forwarded_Proto}|Matches the Pattern |^http$
 Action:
  Redirect to URL: https://{HTTP_HOST}{REQUEST_PATH}
  Redirect Type  : 302

The problem is, we had a powershell script that installed in the web server itself to check whether the website is available or not using [System.Net.WebRequest]::Create($siteUrl). The script reports HTTP 200 (OK) most of the time, but sometimes it fails with 404 HTTP Error. Since the PowerShell script is locally executed, it should not have the X-Forwarded-Proto header and never get 404 by IIS.

Is it possible in any way, that URL Rewrite fired when it should not?