When using the People Picker control in Nintex forms, People Picker might display users that no longer exist.
Ensure that these user accounts have been removed from the hidden list at: http://sitecollection/_catalogs/users/simple.aspx
Review the People Picker scope, as defined by SharePoint. For more information, Configure People Picker in SharePoint 2013 in Microsoft TechNet.
The Nintex Forms people picker is functionally the same as the SharePoint people picker and leverages the same settings and scope. As part of this it will pull user accounts from the SharePoint Hidden Users list at the site collection level.
If a user account still exists in this list they will still resolve in both the Nintex and SharePoint people picker controls.