CLIENTUD by default

I’ve come across this before but I can’t remember how it resolved itself. When creating a new user (just a Member) it adds the suffix ‘_CLIENTUD’ onto the user ID ok but it doesn’t add it to the userDirectory field.
Has anyone else come across this? As a work around I’ve ended up forcing it in the setData() function of dmProfile but it would be good to know if there is a logical reason for this behavior.