Version 1.760 on Debian 8 the Users and Groups list omits some usernames that are in /etc/passwd. Webmin configuration shows that /etc/passwd is the file to use and the user is able to login. It may not be significant but the missing name starts with a single upper case letter. Another username that starts with 3 upper case letter shows in Users and Groups.
Can you give an example of a specific username that is ommitted?