I've just checked your community ( I hope it's for the URL which you've assigned to this topic ) and I think there's some misunderstanding how the sitemap works.
The 1. page (sitemap.php) is the sitemapindex, which contains the links to your sitemaps. If you follow these 2 links, you'll see that all your forums and topics are listed;)
For further information pls see the google docs https://developers.google.com/search/docs/advanced/sitemaps/large-sitemaps