The bug updating automation checks https://opendev.org/openstack/project-config/src/branch/master/gerrit/projects.yaml in order to know where to find bugs for each project, and currently the entry for the openstack/python-mistralclient project says its bugs are grouped with the "mistral" project in Launchpad, so it was expecting bug 1931558 to be for mistral rather than python-mistralclient here. If you push a change to remove the groups entry for it, this would work the way you expect (it would default to looking for python-mistralclient bugs instead of mistral bugs when reporting openstack/python-mistralclient change activity).
The bug updating automation checks https:/ /opendev. org/openstack/ project- config/ src/branch/ master/ gerrit/ projects. yaml in order to know where to find bugs for each project, and currently the entry for the openstack/ python- mistralclient project says its bugs are grouped with the "mistral" project in Launchpad, so it was expecting bug 1931558 to be for mistral rather than python- mistralclient here. If you push a change to remove the groups entry for it, this would work the way you expect (it would default to looking for python- mistralclient bugs instead of mistral bugs when reporting openstack/ python- mistralclient change activity).