Your -arrs see the torrent download folder as /mnt/arr-stack/torrents/completed, and qBittorrent sees it as /downloads.
Maybe this is only a problem with Transmission, but I've had trouble making my Arr stack play nice with torrents when the different apps think downloads live in different folders.
Hey, at least all of us peeps in the US can upgrade our >$100 capped plans to unlimited for the low-low price of $30-50/month (i.e. what some of our friends overseas pay for their whole-ass unlimited crazyfast internet plan).