function custom_format_multiselect( $value, $field_type, $field ) {

    if ( 'multiselect' === $field_type && 'GDPR_Consent__c' === $field ) {

        if ( ! empty( $value ) ) {
            $value = true;
        } else {
            $value = false;
        }
    }

    return $value;

}

add_filter( 'wpf_format_field_value', 'custom_format_multiselect', 20, 3 );