It's not a rule, it's just the way the film industry works. The script says the name of the character in bold black letters and then that's the name you see in films credits. I should know.
You know nothing. What about characters who are uncredited? And you mean to tell me that every credit in every movie has their full official name in the credits? That's literally impossible and easy to prove false.
And the script doesn't mean anything either. There are plenty of scripts that will even change a character's name mid-screenplay to note some kind of transformation or preserve a spoiler or secret.
And don't write "I should know." Especially when you're wrong. You look foolish. That's just the way the Internet message board industry works.