Go to the part where Alex has just "risen" and before he jumps off of the prison. The camera follows Alex, and he has long hair, goes to a close-up, and he has a pretty significant haircut, cut back, and he's got long hair again!
Corvis' hair doesn't actually become shorter and then long again during the jail break. It actually becomes sllicked back with gel. So the real questions is where he got the time to style his hair during the jail break?
His hair also goes from slicked back to unkept repeatedly throughout the movie.
I dont think that it is such a bad slip up. I think it was suppose to be that his hair was burned off, and it healed. I mean if he gets stabbed or shot it heals instantly. That could be what happened with his hair. I don't think that they would have made a slip up that big.