I dont know what is causing but i tried tons of fixes, i mainly use public torrenting, and sonarr and radarr are doing it.
I added *.arj and arj to the quality profile avoid list without success, and prowlarr is as torrent (i’ve read sometimes it gets confused and uses nzbget)
I mean, I’ve only gotten those from one tracker, and haven’t gotten them since I’ve removed that one tracker. Sonarr at least recognizes it’s not a media file and won’t copy it to your media folder. And you can use regex filters with qBittorrent. There hasn’t been a legitimate use of .arj in what, 30 years? So you won’t be missing anything by filtering it.