I noticed the same, I had nodes on 2 different ISPs. On one ISP inbound IPv6 was blocked at router, while on other IPv6 was fully allowed. Tailscale detected

I noticed the same, I had nodes on 2 different ISPs. On one ISP inbound IPv6 was blocked at router, while on other IPv6 was fully allowed. Tailscale detected this is automatically created the tunnel from the blocked one to the other. I was super impressed, as this was handled automatically.