Yes, Digg has tightened the constraints on user names over time. All
active users are available through the API, even when the user name
is one that would not be allowed today.
It's quite common for user activity to remain available after a user
account is no longer active. On Digg.com itself, for example, you'll
see activity by users who no longer have user profiles.
So these API responses are working as intended.
Thanks for clarifying. In that case, let us look into this as a bug.