If you name rep in part "default" it will change name to "default rep". At first its seems you can edit it but then inside assembly you don't see it at all and when you come back its also doesn't work.
The fun thing it exist in few Creo versions I tried. I don't remember that to be issue in ProE. (could be nice if someone who is still working in ProE can confirm)
I'm the only one here who don't like to have master rep as default for parts?