Contact form success message not displaying

I created a contact form with a standard thank you message to be shown upon submission, but it never shows. On the Edit tab of the Contact Form, under the Message field it states: “The message to display to the user after submission of this form. Leave blank for no message.” It seems very clear that the message should be displaying. Any ideas why it’s not?

Drupal version: 


Source: https://www.drupal.org/taxonomy/term/22/feed