How come a subscription administrator not finding a user?
This can happen if the user does not belong the workspace that's being examined. Note in the screen shot in the Question the look up is in the Current Workspace.
If the user has a Default Workspace which is not the Current Workspace, that is either another workspace or an unset default workspace, then it will not be found.
In this screen shot this user does not have a default workspace set:
- Ensure the user has a default workspace set and that the subscription administrator is set to that workspace.
- Alternatively, change the drop-down scope to 'All Workspaces'.