Comment 1 for bug 1754677

I have a feeling the reason for this in keystone is that the query in question isn't looking at the type when doing a query: https://github.com/openstack/keystone/blob/master/keystone/assignment/backends/sql.py#L358

just the other fields:
https://github.com/openstack/keystone/blob/master/keystone/assignment/backends/sql.py#L363-L366

Chances are the same error would occur if you assign to a group a project and a domain scope.

This is most likely legacy code that needed to deal with multiple types of assignments between projects and domains when the two were actually two different types. Now that a domain is a project, the type field is almost pointless in the actual internals, unless there is somewhere that actually makes a distinction between domain vs project scope on an assignment.