Normally the title placeholder changes to ‘What is this discussion about in one brief sentence’ when the topic featured link enabled admin setting has been toggled off. You could check the settings page to see if that’s the case?
I am unsure why one category would still be allowing it though?
If that global admin setting is enabled it should allow it for all categories at the point of topic creation. Just to check, are you having this issue when creating a topic, or when editing a topic?
I’ve just had a quick run-through on my test site, and when I create a new category it does at first give the ‘What is this discussion about in one brief sentence’ placeholder text for all its new topics until I refresh the page to reload the site. Could you try refreshing your page and trying those categories again and see if that helps?