To have the possibity to use inherited categories, you have to operate the following changes into issue.php file

replace (line 76):

if( $this->keepCategory_ ) {
$t_category_id = category_get_id_by_name( $reader->value, $t_project_id);

if( $t_categoryid !== false ) {
$this->newbug->category_id = $t_category_id;
}

}

by :

if( $this->keepCategory_ ) {
$t_category_id = category_get_id_by_name( $reader->value, $t_project_id, false );
if( $t_category_id === false ) {
$t_category_id = category_get_id_by_name( $reader->value, 0);
}

if( $t_categoryid !== false ) {
$this->newbug->category_id = $t_category_id;
}
}