Go to the settings of some topic and check “Suppress category from latest topics.” then post something to that topic. It is correctly not shown in the Latest tab, but when you open the RSS feed then it contains the post.
Can be seen here:
Projects in Copr - Fedora Discussion
this category has “Suppress category from latest topics.” set to on. There is a lot of new topics.
does not show those topics, however
shows topics from Projects in Copr category.