Custom Validation not working (anymore)

I’m still have problems with the custom validation.

According to the tutorial, this should work. But it seems $result->invalidate is not working with a simple custom from tag.

Tutorial I used (together with the previous article): https://contactform7.com/2015/03/28/custom-validation/

Here is the testcode I created in my plugin:

add_action( 'wpcf7_init', 'rw_add_form_tag_kursdropdown' );
 
function rw_add_form_tag_kursdropdown() {
		wpcf7_add_form_tag( 'kursauswahldropdown', 'rw_kurs_dropdpow_form_hander' );
}
 
function rw_kurs_dropdpow_form_hander( $tag ) {
	$atts = array(
		'type' => 'text',
		'name' => 'kurs',
	);

	$input = sprintf(
			'<input %s />',
			wpcf7_format_atts( $atts ) );

	return $input;
}

function rw_kurs_dropdown_validation( $result, $tag ) {
	if ( 'kursauswahldropdown' == $tag->type ) {
		$result->invalidate( $tag, "Are you sure this is the correct address?" );
	}

	return $result;
}

add_filter( 'wpcf7_validate_kursauswahldropdown', 'rw_kurs_dropdown_validation', 1, 2 ); 

I testet it, the condition is met, so the field should be invalid. However nothing happens and the result seems to be unchanged.

What did I do wrong?

Thanks in advance!