Form v7.1.2 basic-catcha field validation ignored

Hi. Following the advice here…

…my form renders the field with the placeholder “copy the 6 characters”, but leaving the field blank (or typing the CAPTCHA incorrectly) doesn’t forestall form submission. Can anyone explain what I’m missing? I did included:

    process:
        basic-captcha:
            message: 'Humanity verification failed, please try again...'