When in the LS421DE Bittorent web gui, you can certainly add a torrent with a magnet link. I've been doing that for a couple of months now. I have NOT hacked my firmware. I stumbled on this post looking for a solution to why the Bittorrent client just STOPS at random times. Well, it doesn't stop, but the peers/seeds drop to zero. I have to go into the NAS admin and disable and re-enable the Bittorent service with the little slider switch.
That gets the torrents running again....until I have to repeat the process. Usually about five times a day. If I remember.
Not sure WHY it's not updated!
LS421DE user myself, with 300 torrents loaded!
The situation you describe is actually more of an issue with the network, if I have active torrents, reset my router, and the IP or UPnP clear or change in the router, all current torrents will show seeding/downloading, but not actually seed/download!
To resolve this I set a reservation in my router for the LinkStation and set a manual Port Forward instead of using UPnP, this doesn't TOTALLY resolve the issue, but unless I restart my Router in quick succession, the torrents resume just fine afterwards
Long story short, this is a mix of networking issues with the Embedded Torrent client (which while not being the latest, is still very up to date compared to the older LS model's).