sanitization_cb in PHP class

Re: https://github.com/CMB2/CMB2/wiki/Field-Parameters#sanitization_c

$opts->add_field(
	array(
		'id'              => $this->field_prefix . 'item_class',
		'name'            => __( 'Individual CSS Class', 'my-text-domain' ),
		'type'            => 'text_medium',
		'sanitization_cb' => array( $this, 'sanitize_css_classes' ),
	)
);

and then, in the same PHP class:

protected function sanitize_css_classes( $value, $field_args, $field ) {
	$classes = explode( ' ', $value );

	$output = '';
	foreach ( $classes as $_v ) {
		$output .= ' ' . sanitize_html_class( $_v );
	}

	return $output;
}

$this->sanitize_css_classes() is not getting called.

Shouldn’t this sanitization_cb work?

Thank you for the assistance.