The same happened to me. Out of the 6 people in my friends list, only 2 had their normal names. The two people were Cringeh and ThePumbaanator, so since Cringeh's name is fine on your friends list, it could be that certain people don't show up as UnknownUsername.
People that showed up as UnkownUsername for me was Epicsmiley, _pozer_, DDENGAR, and my alt.