Every forum should always enable the 'Use full friendly URLs' feature. This will remove the index.php part of the URL to your forum. It will also ensure Google can use the sitemap.xml file properly.
Admin control panel -> Setup -> Options -> Basic board information -> Use full friendly URLs
Note: Regarding the sitemap.xml issue, see this thread https://xenforo.com/community/threads/confused-by-sitemaps.197291/post-1531278
Admin control panel -> Setup -> Options -> Basic board information -> Use full friendly URLs
Note: Regarding the sitemap.xml issue, see this thread https://xenforo.com/community/threads/confused-by-sitemaps.197291/post-1531278