Just an observation:
if the responsible (userxyz) does not exist, ALL active assignments on the server are returned.
This may be intended, but more intuitive would be to return nothing, because a non-existing user cannot have any assignments.
The behavior described above by @klaus is not intentional.
We will fix this. The direction of the fix is: to return an empty list in the case when:
userxyz does not exist;
userxyz is defined on the server, but is in a different workspace (not visible to the account, from which the query is being submitted).