After changing USERGROUPS_ENAB to yes in the /etc/login.defs file you've changed the behavior, as you are wanting, for the useradd command defaults. So, for example, you could run this command as root and it will do what you are expecting:
linux-54pe:~ # grep "USERGROUPS_ENAB" /etc/login.defs
USERGROUPS_ENAB yes
linux-54pe:~ # useradd bob
linux-54pe:~ # cat /etc/passwd | grep bob
bob:x:1003:1003::/home/bob:/bin/bash
linux-54pe:~ # cat /etc/group | grep bob
bob:!:1003:
The problem is that you are using YaST2. YaST2 is using its own default group assignment and so it doesn't respect the default changes made to useradd. In the /var/log/YaST2/y2log you can see that when I attempted to create the user frank:
2017-04-25 10:44:02 <1> linux-54pe(2871) [Perl] modules/Users.pm(Users::CommitUser):3517 commiting user 'frank', action is 'add_user', modified: 1, ldap modified: 0
2017-04-25 10:44:02 <1> linux-54pe(2871) [Perl] modules/Users.pm(Users::CommitGroup):3787 commiting group 'users', action is 'user_change_default'
Also, in the YaST2 module when you are creating the user in the Details tab you can see at the bottom that it's assigning it to its own default group parameter of users.
screenshot showing parameter
If you have a support entitlement with SUSE you can contact them to see if they are willing to submit this as a bug. At the very least they should be able to put this in as an enhancement request.