Wow I didn't realize transmission had its own implementation of the torrent protocol, I thought they used libtorrent like everyone else. According to a comment

Wow I didn't realize transmission had its own implementation of the torrent protocol, I thought they used libtorrent like everyone else. According to a comment on that issue it seems that not only does it not support v2, but it also breaks on opening v2 .torrent files. Lots of torrents are generated manually using qBittorrent, i.e. private trackers, this means that torrents might start breaking for transmission users soon.