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’ve only seen it for two so far, and after I blocked them it was fine.
Part of the problem was Sonarr trying to grab episodes that hadn’t aired yet. I think a fix for that is forthcoming. That should cut down on them. I don’t know if there is a fix to let you block extensions.