Order of events...

1. Looked up IP
2. Connected to Tailscale, set exit node through Mullvad
3. Looked up IP again, was different
4. Started seeding in the background while working on other stuff
5. At one point I saw Tailscale icon flicker
6. Later I got an angry email from my ISP with a timestamp that lined up

Been seeding for years, and this was my first leak. Was for a recent popular film (Linux ISO) that I've been seeding for a year. I contacted Tailscale support to express my concern. This is what they said...

Though we have an open feature request for this (link), I don’t believe there are current plans to add a killswitch to the client for Mullvad.

If this is something that is important to you, the quickest solution to this would be to purchase a Mullvad subscription directly from them, since their client has a number of features more geared towards tightening users online privacy – including a killswitch.

So I suggest not using Tailscale with Mullvad for such purposes. I don't think this is a priority for them. For other uses it's been fine.

I imagine this could have been avoided with a restrictive torrent client configuration, as is typically recommended online. I've tried and failed to get that working in the past. I'll try again once I change out my VPN. If you've been putting that off, learn from my mistake and look into it!