I misread your OP, it sounded to me like it was working the way it’s supposed to work.
I just did a test and can confirm that’s it is working correctly on my test site.
The validation is done using JavaScript and AJAX. Any error in JS or during the AJAX request can cause the request to fail and this might cause what you’re seeing.
The first step is to deactivate other plugins and possibly switch themes to narrow down what might be causing the conflict.
