I added one (only one to test it as you can see) subforum but I didn’t added the code or the option in Tiki.
But hey, that’s dogfooding ! (bug report created: https://dev.tiki.org/item7765-Tablesorter-filter-is-misplaced-in-the-forum-should-be-below-the-subforum-list)
Actually the tablesorter is doing the right thing.
BUT you and I expect it look at the discussions list and not the subforums list.
It is misplaced and should be just above the discussion list (table).