Are you running Jellyfin on the NAS as well as the arrr stack?
Also which Synology NAS do you have exactly and as someone else asked, what client / box are you using to access it with?
I’ve got a couple of Synology boxes and only run Radarr/Sonarr/Transmission on them and that’s it - Jellyfin is on another SFF PC and I’ve also been thinking of taking Transmission out of the Syn boxes since it nails them on large & multiple downloads.