As far as I know, dots where never supposed to be valid characters for Unix usernames. The historical reasons are many, including the syntax of some commands that take username as a parameter. Several commands will that username.group as a parameter and, as you can see, the dot can get in the way of that.
So, if you ask me, the upstream provider just corrected a long standing compatibility issue.
I think it's really a bug :
https://bugzilla.redhat.com/bugzilla/show_bug.cgi?id=159034
but not something the upstream provider cares to fix it seems
In any case, if you really want to have usernames with dots on it, you can create them manualy, or create them without the dot, and then change them by editing the apropriate files.
That's what I did now but I'm not very comfortable in editing /etc/shadow by hand
Thanks for the feedback!