andriusrimkus

(function($) {
    $(document).ready(function() {
        jQuery(".ninja-forms-form").on('submitResponse.resetCaptcha', function(e, formData, jqForm, options){
            if (typeof grecaptcha !== 'undefined') {
                // add error manually if captcha isn't solved
                if (!grecaptcha.getResponse()) {
                    jQuery("#ninja_forms_field_8_error").prop("innerHTML", 'Please confirm that you are human');
                }

                // reset captcha
                if (grecaptcha.getResponse()) {
                    grecaptcha.reset();
                }
            }
        });
    });
})(jQuery);

A slightly improved version.

This is my solution to the problem:

(function($) {
    $(document).ready(function() {
        jQuery(".ninja-forms-form").on('submitResponse.resetCaptcha', function(e, formData, jqForm, options){
            if (grecaptcha && grecaptcha.getResponse()) {
                grecaptcha.reset();
            }
        });
    });
})(jQuery);

It resets the captcha state (if it was activated) after submitResponse event. This is a solution for ajax powered form.