Maybe you can try to cleanup the file association in KDE Settings → Applications → Files Association? expand categories, click on the first one and with down arrow key review each one and when you see qbittorrent on right panel you remove it. Just a suggestion, it can be pretty time consuming.
Also maybe simply try to set the default association in Dolphin for all these files you see qbittorrent in right click menu, select open with another… chose the application and tick the box Remember file association.
I know I had the issue before, and I think I did a little bit of both methods, and if I look at it now I don’t see qbittorrent anywhere in right click menus except for torrent files.
I may have also reset KDE at some point, deleting the major user config files, so it may be it something along those lines, for an unrelated problem:
cd ~/.config
mv *rc plasma-old/
mv kdeglobals plasma-old/