Right now, the script we use to rewrite /etc/network/interfaces for bridging doesn't pay attention to included subdirectories (/etc/network/interfaces.d/*). On many clouds, cloud-init creates 'eth0' in /e/n/i/50cloud.cfg
Which means that the Controller may decide "I want you to bridge eth0", and trigger the bridge script to run, but it won't actually be able to create br-eth0, but doesn't actually report a failure. Which means we continue, and then fail late when we try to use br-eth0 but it doesn't exist.
This is not a critical issue today, because for clouds that do use /e/n/i/50cloud.cfg we don't support getting IP addresses from the underlying cloud anyway. If/when we do, we'll need to fix this.
We'd like to move away from a Python script anyway, so maybe we can just do it there.
We don't use e/n/i for new deployments, and certainly the Netplan bridger works with config in the xxx.d form.