From TS support
"I’m Kelly from the Tailscale support team. Thanks for reaching out! This is a common point of confusion- Even with the “Key Expiry: Disabled” option selected in the Tailscale web UI, that only applies to machines authenticated via the web login.
You need to generate a Reusable, Ephemeral = false, Pre-Auth Key via the Tailscale admin panel, and use that on the pfsense."