I tracked down how I figured it out - and then promptly forgot until I had some time to check my post history today - yay holidays! 
So there is a post here about redirecting your tailscale clients through the router and through Adguard - i.e. so you get adblocking on the go Tailscale - Using AdGuard DNS is not working - #5 by Jopie7863
There is a part where the staff member says:
”Note: Currently you might currently observe that Tailscale is rebooting itself, resulting in a repeated "up" and "down" cycle.
To address this, please click "Stop" when this button is available. This will allow Tailscale to have a break to reconnect and create the tailscale0 device, and then make this interface work again.”
So that’s the best I can do in terms of figuring out what’s going on and why the ‘Stop’ works!
Hope that helps.