This bug seems to be by design.

project show( maybe other command also ) uses find_resource() method in openstackclient /common/ utils.py.
https:/ /github. com/openstack/ python- openstackclient /blob/master/ openstackclient /common/ utils.py# L52

find_resource() method first tries to find by name_or_id. If it failed, list with query parameter of name_or_id.

project show --parents is the following:
If --parents option with project name is given, first try to find by project name with parents_as_list query paramter and fail. Then list project with query parameters of project name and parents_as_list.
Since list projects RESTful API does not accept query paramter parents_as_list, simply show project by query parameter of project name.