Last modification date not updated

When I edit a node, and run cron, last modification date of the node is not updated.
Taking a look at the sources it looks like xmlsitemap_node_xmlsitemap_links function only copies the content of xmlsitemap_nodes table to xmlsitemap table when priority node or language has changed (or if the node doesn't exist on xmlsiteamp table previously).
If I manually delete the content of xmlsitemap and the cached files, the first access to http://mydomain.com/sitemap.xml updates last modification date of all the nodes, but I don't think this is the right way...Is this the expected behaviour of the module? Am I doing something wrong?