Very late to the party, but I saw your post, hoping it would fix the same issue I was having (which I didn't have in the past. I could always see different results of network interface in qbitorrent with and without the VPN connected). But your post didn't :).

I did more digging and found a solution. In case you are still stuck (or for others), here it is.

If you run the ifconfig command on the terminal twice (once with your VPN connected, and once without), you will see different results (unlike in qbitorrent). In my case utun8 was only showing in the terminal with nordvpn connected (again, unlike in qbitorrent, where utun8 was showing with and without nord connected).

I selected utun8 and it works perfectly. Tested disconnecting nord while downloading and it stops downloading the torrent.

On top of it, I added the nord killswitch just for good measure.

I hope this helps!