If you added the die statement into the correct location then this should not be conditional, if the server behavior is triggered then this die statement should occur and at the very least make the page display blank.
Are you still getting to your index page? If so this tells me that you have a redirect occurring before the update, or that the update trigger is not valid. If you are using this same trigger in any of the other server behaviors on the page and can confirm they are working correctly then I would say it is a redirect that is causing the update to not occur. If you can post back with your entire page in a zip archive or pasted into a php /php code block that would be very helpful for looking for the cause of this.
If you want to do another quick test of the update server behavior you can add in a die statement just after the trigger like this:
// WA Application Builder Update
if (isset($_POST["LogIn_x"])) // Trigger
die("inside of the update");