The fact that this isn't been added causes an exactly issue. I created a bug report for this.

When a customer doesn't enter a reason for canceling the service and clicks submit, the page reloads. The previously selected choice between 'Terminate immediately' and 'End of billing period' is then also reset. An inattentive customer then accidentally cancels the service with the 'Terminate immediately' option because their earlier choice of 'End of billing period' has been restored.