What do you get if you run:
lxc exec ubuntu-master -- ping 8.8.8.8
I noticed that your host has a docker0 interface, suggesting that you have docker installed, which is well known to apply firewall rules that prevent LXD lxdbr0 traffic from accessing external resources.
This normally manifests itself in the container not get IP addresses via DHCP, but in this case it may be manifesting itself as preventing traffic being forwarded through the bridge.
Please take a look at Lxd and Docker Firewall Redux - How to deal with FORWARD policy set to drop and let me know if that fixes it.