I tested this again against OSP 11 (Ocata):
~~~
[stack@undercloud-8 ~]$ openstack user list --domain redhat
+------------------------------------------------------------------+----------+
| ID                                                               | Name     |
+------------------------------------------------------------------+----------+
| 80fc5502450b25ec0691dccc27307efc65ebb5c7f54db131e120f4a8bb6cecf0 | svc-ldap |
| 3edee38198325e08fd8795862575196802ac7572bc1b00d0f0dade56d674a7e3 | akaris   |
| f612704506d90104aebdc0989f6120a8db166ad0e80b9990262ea32487209d31 | nalmond  |
| 6992dec6afa26e9faac16e0f56748a6b492ada373c21eaad919af87cc89a831e | test     |
+------------------------------------------------------------------+----------+
[stack@undercloud-8 ~]$ openstack role add --project demo --user 6992dec6afa26e9faac16e0f56748a6b492ada373c21eaad919af87cc89a831e _member_
[stack@undercloud-8 ~]$ openstack role assignment list --names
+---------------+------------------------------------+-------+-----------------+------------+-----------+
| Role          | User                               | Group | Project         | Domain     | Inherited |
+---------------+------------------------------------+-------+-----------------+------------+-----------+
| admin         | nova@Default                       |       | service@Default |            | False     |
| admin         | aodh@Default                       |       | service@Default |            | False     |
| admin         | glance@Default                     |       | service@Default |            | False     |
| admin         | heat-cfn@Default                   |       | service@Default |            | False     |
| admin         | swift@Default                      |       | service@Default |            | False     |
| _member_      | test@redhat                        |       | demo@Default    |            | False     |
| ResellerAdmin | ceilometer@Default                 |       | service@Default |            | False     |
| admin         | ceilometer@Default                 |       | service@Default |            | False     |
| admin         | panko@Default                      |       | service@Default |            | False     |
| admin         | gnocchi@Default                    |       | service@Default |            | False     |
| admin         | heat@Default                       |       | service@Default |            | False     |
| admin         | neutron@Default                    |       | service@Default |            | False     |
| admin         | admin@Default                      |       | admin@Default   |            | False     |
| admin         | placement@Default                  |       | service@Default |            | False     |
| admin         | cinder@Default                     |       | service@Default |            | False     |
| admin         | heat_stack_domain_admin@heat_stack |       |                 | heat_stack | False     |
| admin         | admin@Default                      |       |                 | redhat     | False     |
+---------------+------------------------------------+-------+-----------------+------------+-----------+
[stack@undercloud-8 ~]$ openstack user list --domain redhat
+------------------------------------------------------------------+----------+
| ID                                                               | Name     |
+------------------------------------------------------------------+----------+
| 80fc5502450b25ec0691dccc27307efc65ebb5c7f54db131e120f4a8bb6cecf0 | svc-ldap |
| 3edee38198325e08fd8795862575196802ac7572bc1b00d0f0dade56d674a7e3 | akaris   |
| f612704506d90104aebdc0989f6120a8db166ad0e80b9990262ea32487209d31 | nalmond  |
+------------------------------------------------------------------+----------+
[stack@undercloud-8 ~]$ openstack role assignment list --names
Could not find user: test. (HTTP 404) (Request-ID: req-62df0690-1e42-4fd3-9655-434540137946)
[stack@undercloud-8 ~]$ openstack role assignment list | grep 6992dec6afa26e9faac16e0f56748a6b492ada373c21eaad919af87cc89a831e
| 9fe2ff9ee4384b1894a90878d3e92bab | 6992dec6afa26e9faac16e0f56748a6b492ada373c21eaad919af87cc89a831e |       | cda9085fa7c7441b9c08ccf4ca09f690 |                                  | False     |
[stack@undercloud-8 ~]$ openstack role list
openstack r+----------------------------------+-----------------+
| ID                               | Name            |
+----------------------------------+-----------------+
| 26fe0e40514b424cb00afd6fb5cd7789 | ResellerAdmin   |
| 904df4d9f45949179df5b6f60ce63552 | heat_stack_user |
| 9fe2ff9ee4384b1894a90878d3e92bab | _member_        |
| b8366308a04d4649bf9824f12c0194d5 | swiftoperator   |
| d5f53263bfa6429294b9cd4cd13d7ea2 | admin           |
+----------------------------------+-----------------+
[stack@undercloud-8 ~]$ openstack role remove --project demo --user 6992dec6afa26e9faac16e0f56748a6b492ada373c21eaad919af87cc89a831e 9fe2ff9ee4384b1894a90878d3e92bab
No user with a name or ID of '6992dec6afa26e9faac16e0f56748a6b492ada373c21eaad919af87cc89a831e' exists.
~~~

Same issue in Ocata.