Leon’s hair physics have perplexed me since day 1 so I decided to do some research, and this is what I came up with.
In most of the games and movies, especially 2 and 6, it’s kinda thick hair so it holds its shape pretty well. In Damnation it’s this… slick curtain type hair that’s very straight, and I guess it just acts the way long hair does if he’s upside down. According to my research (images of kids on monkey bars) his would usually somewhat keep its shape at that length.
On top of that, it’s an UNDERCUT so it gets all the more wonky at any orientation other than upright.
In any case, he’s probably gotta use so much hair product to keep it out of his eyes in the first place, so…