I believe this is related to the fetch artifact refactor. The incorrect file is probably being fetched from upstream.
I have checked the ami_override.yml and it does not contain the current_asgs key (why would it, this is generated by the deploy stage!!)
it seems that the incorrect file is being passed to the script, the ami_deploy_info.yml script from the deploy stage should be used.
Looking deeper on this it seems that the ami_id that is in the ami_deploy_info.yml file would be used to deploy an AMI if the prior ASG was not available, but this does not seem correct. That ami_id is the newly deployed AMI not the AMI that was previously deployed.