The update prepare command calls the get_config workflow from workbooks/package_update.py. This workflow calls the tripleo.config.get_overcloud_config action, but it only passes in the container name, not the container_config name. This means the action ends using overcloud-config, even if the stack name is called something not overcloud. This can cause issues with multi-stack and the multiple overcloud features, or just using a non default overcloud name.
Since the problem described in this bug report should be resolved in a recent advisory, it has been closed with a resolution of ERRATA. For information on the advisory, and where to find the updated files, follow the link below. If the solution does not work for you, open a new bug report. https://access.redhat.com/errata/RHEA-2020:0283