Missing Response Messages

All of a sudden, people are submitting forms multiple times because there is no response when they submit. The messages are being sent but they get no visual confirmation. When I test page and inspect the page I see this in the generated source code:

<div class="wpcf7-response-output wpcf7-display-none"></div>

and when I submit the form it changes to this:

<div class="wpcf7-response-output wpcf7-display-none wpcf7-mail-sent-ok" style="display: none;"></div>

Where is the message content and why is the output styled display:none ?

I tried deactivating all other plugins and changing to a default theme. Same problem.

https://wordpress.org/plugins/contact-form-7/