When you send general emails from the People page or a Team page and you choose a template, the title of the template becomes the subject of the email. When you send an email from a plan, instead of using the template title, the email subject defaults to the plan date, and ignores the title of the template. Today's update provides a new option for Scheduling Emails and Signup Sheet Emails to use the title of a template as the email subject.
When you are editing Scheduling or Signup Sheet email templates, you can now check the box to "Use as subject." When you compose an email from a plan and choose this template, the subject will be replaced with the title of this template. When that box is not checked, the subject will be the plan dates.