Is deleting a cookie in Javascript Synchronous?

I have more or less a duplicate of Javascript Delete Cookie Before Reload or Redirect which never got any answers.

I inherited some javascript code that does the following

 function delete_cookie ( cookie_name )
  {
     var cookie_date = new Date ( );  // current date & time
     cookie_date.setTime ( cookie_date.getTime() - 1 );
     document.cookie = cookie_name += "=; expires=" + cookie_date.toGMTString();
  }
  delete_cookie ( "sessionId" );
  window.location ="https://some_redirect";

The problem is that the GET for the redirect page contains the original cookie (which apparently was not deleted), and the server just continually responds with the delete/redirect code (tested on chrome/canary/IE). If it makes a difference, the cookie was initially set to simply "sessionId=;"

The cookie is always set with the following pattern

"Set-Cookie: sessionId=%s;path=/;%s postId=%s;%s " "\r\n"

I am aware that toGMTString is deprecated, and that setting directly to window.location is perhaps non-ideal. Neither of those seem related to the problem.

If I put the window.location call inside a setTimeout, or if I execute it as a callback from delete_cookie then everything magically works perfectly. But why?

This seems to work reliably

function delete_cookie (cookie_name, callback)
{ 
   ...
   callback();
 }
delete_cookie( "sessionID", doRedirect);

where doRedirect is just the window.location

Is this behavior expected? My presumption was that when the cookie was set with an expiry date in the past it would be deleted before code execution continued. Is there a better mechanism to ensure the cookie has been deleted prior to the redirection?

EDIT: Additional information

Modifying the cookie as follows doesn't help things

       function delete_cookie ( cookie_name )
       {
+         var retries = 0;
          var cookie_date = new Date ( );  // current date & time
          cookie_date.setTime ( cookie_date.getTime() - 1 );
          document.cookie = cookie_name += "=; expires=" + cookie_date.toGMTString();
+      
+         while (document.cookie.indexOf("sessionId=") != -1)
+         {
+            retries +=1;
+            if (retries == 1000) 
+            { 
+                console.log("giving up after 1000 retries");
+                break;
+            }
+
+         }
+         console.log("We had to retry "+retries+" time(s)");
       }
       delete_cookie ( "sessionId" );

Console output is

giving up after 1000 retries

We had to retry 1000 time(s)

Interestingly, if I am debugging on the client I see the following

Canary (Version 42.0.2302.2 canary (64-bit))

After the javascript redirect code is served up from the dev console "Resources" tab, under Cookies->10.0.11.118 it says "This site has no cookies."

But in a separate tab to chrome://settings/cookies I can still see

Name:   sessionId 
Content:     
Domain: 10.0.11.118 
Path:   / 
Send for:   Any kind of connection 
Accessible to script:   Yes 
Created:    Thursday, February 12, 2015 at 12:26:07 PM 
Expires:    When the browsing session ends