Nick Diaz Strikeforce alter ego inaccessible

The Nick Diaz Strikeforce alter ego was released as part of the UFC 307 fight week challenges. To unlock him you simply had to have 5 welterweight fights, which I am very certain I did. But still I have not been able to unlock him. What’s even more disappointing that now that the fight week is over, there is absolutely no way for me to unlock him or even purchase him. He just remains locked. Is there any way we can purchase alter egos later on in the game after they have been part of a particular challenge? Or are they simply locked forever?