Your tailscale deb package has a dependency on iptables
(Ubuntu 20.04, focal)
This results in all the legacy handling to be installed, such that
/usr/sbin/iptables links through to
tailscale up then adds your own chains,
ts-forward and ts-input
As it happened, on one of my machines I had been teaching myself nftables, and had intentionally removed iptables etc, ufw and had created my own nftables.conf
Perhaps I am unobservant or inexperienced, but I missed having all iptables put back and it caused things not to work until i removed nftables.
I would have liked to be warned!
So, I conclude at the present time Tailscale is incompatible with pure only nftables
unless you can let me know the work around.