I just did a quick web search and found several topics and posts from different years which state, that Nord VPN does not do port forwarding.
https://github.com/haugene/docker-transmission-openvpn/issues/1375
https://www.reddit.com/r/nordvpn/comments/mj0y0v/linux_ports/
https://www.bleepingcomputer.com/vpn/guides/nordvpn-for-torrenting/
Also other users reported bad transmission speeds with NordVPN.
Is it possible, that before the last update your torrent connections were not using the tunnel at all?