No wonder, I recently installed the latest sonoma beta and couldn't for the life of me get Mullvad to work. Glad to hear Mullvad is working on a workaround. I even considered downgrading back to Ventura this morning. I feel validated!
I assume the tailscale/mullvad stuff still works. Might be a nice workaround until apple gets it fixed.
Went to look at tailscale vpn and didn't realize it was a service entirely contained within tailscale, so I can't use my existing Mullvad account or credit tailscale with my already-purchased Mullvad credits :(
Tailscale's business model has never been anonymity, last I used them they even required either a Google or O365 account to even use the service.
You might want to look into Headscale[1] instead, it's a server-side implementation of Tailscale that you can self-host. It comes with its own drawbacks but from what I've seen it's worth it if you want more control over the network.