My LXD install is via snap and so I got auto-upgraded to the 3.0 release. Which broke my networking.
The fix for me was to create a new network interface and move all of my containers over to it and delete the old one. Wasn't hard once I figured out what was going on, but took me a while. Hope that save someone time.
Also, if you're using the snap I'd recommend switching from "stable" and to a release stable. It upgrades less. I'm now on the "3.0/stable" branch.