In the /etc/group file, a user does not need to be added to its primary group. The connection is established in /etc/passwd only. So, if my primary group is 'users' and secondary, 'work' I will only see the username appended to work in the /etc/group file.

I believe this was done to limit the length of lines in /etc/group quite some time ago. However, it is nothing to worry about and the system is behaving as expected.