While doing some routine patching this weekend, I noticied one of my Ubuntu servers was not properly updating Tailscale. This was one of two identical Ubuntu servers running Pihole to handle DNS. One was updating, one was not.
Had a look in /etc/apt/sources.list.d/ the working one had the tailscale.list, the non-working install had tailscale.list.distUpgrade. Seemed simple to fix just run:
cd /etc/apt/sources.list.d/
sudo mv tailscale.list.distUpgrade tailscale.list
And then check for updates.
sudo apt update && sudo apt upgrade -y
Should see Tailscale then update.