Google Tag Manager Cookie being stripped.

Hey there,  We have a cookie, setup in Google Tag Manager, which is supposed to follow a user through the site, and expire after 1 month.  

I.e  user clicks on an add,  goes to https://buildertrend.com/?source=dev-test.  Source Tag fires.
user then goes to any other page on the site, we lose the cookie value. Not sure why it is stripping the value ?

<script>
  
  //Get value from URL parameter "source"
  
 function getParameterByName(name) {
     name = name.replace(/[\[]/, "\\[").replace(/[\]]/, "\\]");
     var regex = new RegExp("[\\?&]" + name + "=([^&#]*)"),
         results = regex.exec(location.search);
     return results === null ? "" : decodeURIComponent(results[1].replace(/\+/g, " "));
 }
  
 // Set Cookie
 
 var cookieName = "sourcetag"; // Name of your cookie
 var source = getParameterByName('source'); // Value of your cookie
 var expirationTime = 2592000; // One month in seconds
 expirationTime = expirationTime * 1000; // Converts expirationtime to milliseconds
 var date = new Date(); 
 var dateTimeNow = date.getTime(); 

 date.setTime(dateTimeNow + expirationTime); // Sets expiration time (Time now + one month)
 var date = date.toUTCString(); // Converts milliseconds to UTC time string
 document.cookie = cookieName+"="+source+"; expires="+date+"; path=/; domain=." + location.hostname.replace(/^www\./i, ""); // Sets cookie for all subdomains

</script>

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