Comment 1 for bug 1824435

It's because the context doesn't have a project_id so the first pass creates a record with project_id=None:

https://github.com/openstack/nova/blob/03322bb517925a9f5a04ebdb41c3fd31e7962440/nova/db/sqlalchemy/api.py#L4037

There is also a unique constraint on project_id/name/deleted:

https://github.com/openstack/nova/blob/01c33c584f7209ae02bc537adea85502d52f1c22/nova/db/sqlalchemy/models.py#L660

The migration is just using an admin context which doesn't have a project_id:

https://github.com/openstack/nova/blob/f853e04cd22acc36c906ffb29084160fa05d1ebd/nova/cmd/manage.py#L715

https://github.com/openstack/nova/blob/f853e04cd22acc36c906ffb29084160fa05d1ebd/nova/context.py#L292