Sitecore 8.1 here, I'm currently using the Sitecore AD Module 1.3.
When I search through my users from the backend using the User Manager, I can easily find all the users I want.
When I try to retrieve the from my code though, using UserManager.GetUsers(), some users - seemingly at random - don't show up at all.
Why is this happening? Is there a configuration field I'm not aware of that makes so that they don't show up when I invoke GetUsers()?