I used vBulletin with vBseo before which regulary transfered sitemaps to Google. XenForo seems not to transfer sitemaps, says the Google webmaster tools search console.
Is it necessary to to something to activate sitemaps?
You need to explicitly submit your sitemap via webmaster tools to see the details about it within it. Submit the URL once (the sitemap.php URL) and it should start showing when it updates.