@carabuddy torrents will only count as completed if you select all files and download 100% of the torrent. If you leave anything unselected, after you finish downloading the torrent client will show it as seeding but it will still be occupying a download slot on GT server. All your downloaded/uploaded amounts will still count as usual, there's nothing to worry about. But if you do it too many times eventually you'll run out of download slots.
Just to be clear, not a qbittorrent problem, and not even a problem at all, that's just how private trackers work.