Account creation emails not being sent but other emails are
I would create a clean Drupal site on Pantheon and configure just SendGrid.  Do all emails get sent as expected from the new site?  If so, it's likely that there is some custom code running on the problematic site calling hook_mail_alter or altering the normal mail handling in some other way.

You'll also find some great tips to help troubleshoot email issues in http://www.webomelette.com/drupal-modules-debug-emails