Comment titles are just noise to me, and forcing the users to enter a title above their comment when replying to a comment is cementing this extra noise by force. We have threading already, why would we even need titles? No other bug tracker that I know of (Bugzilla, GitHub, GitLab, Phabricator, etc.) does this.
Okay maybe some folks want titles here, especially when starting a whole new thread, then in that case maybe enforce the title field only for the first post in new top-level threads, but not for follow-up replies. Because when you enforce that, people just make up random/inconsequential titles, or split their message into two sentence halves that are started by the title and completed by the comment, which is silly, and reminds me of people who send me emails where they put everything in the subject line and nothing in the body 🤭
Having more titles than strictly needed adds visual noise and uses precious vertical space, which means more scrolling across long threads.