So I've noticed that if I split a thread, the new thread only exists in the recent activity or if I search it. Doesn't appear in the actual forum that it's split into - in my case, the same forum that the original thread was in.
FYI - just checked the database and solved it. Solved is set to NULL when everything else was 0. Changing it from NULL to 0 then shows the thread in the forum list.
You can do anything