So based on Serge's observation that removing /etc/network/interfaces.d/* fixes this, and the log message
[CLOUDINIT] stages.py[INFO]: Applying network configuration from fallback: {'version': 1, 'config': [{'name': 'lxdbr0', 'mac_address': 'fe:68:41:8c:e7:dc', 'subnets': [{'type': 'dhcp'}], 'type': 'physical'}]}
I suspect that cloud-init is adding a /etc/network/interfaces.d/ configuration for lxdbr0, which it shouldn't do?
So based on Serge's observation that removing /etc/network/ interfaces. d/* fixes this, and the log message
[CLOUDINIT] stages.py[INFO]: Applying network configuration from fallback: {'version': 1, 'config': [{'name': 'lxdbr0', 'mac_address': 'fe:68: 41:8c:e7: dc', 'subnets': [{'type': 'dhcp'}], 'type': 'physical'}]}
I suspect that cloud-init is adding a /etc/network/ interfaces. d/ configuration for lxdbr0, which it shouldn't do?