They’re had to be a reason why this was the case. If you look in a lot of the Epilogue mission images that can be found in the pause menu. So many of them depict John with his original hair. The obvious implication of this being that these images are likely a screenshot from an older version of the game where John kept his hair, instead of using a black version of Arthur’s. As seen in the image below.

So frankly, what the hell happened? Why did the developers apparently remove John’s original playable hair? Considering the game was in development for eight years, you think that would’ve been plenty of time for them to add hair growth to John’s original hair and make it playable but evidently they couldn’t do that.