I dunno if it's related to swing hitboxes, but I also noticed that swinging a sword while mounted seemed a bit off. I've been playing almost the entire game with a menavlion I found pretty early on or a custom menavlion I built. 2h polearms that you can swing is currently OP, I can one shot anything in the game with it or if I don't get enough speed or the target is heavily armored it's a 2 shot.
I have no problems in Warband but I can't hit shit with a sword in Bannerlord (when mounted). It's one of my biggest gripes. It looks to me like it is not just the hitbox but the swing angle is off. It's like you have to look down too far (angle) and the the window is really narrow (hitbox).
It also feels like the damage I do is a little weak (ride by full speed and swing sword at head does not kill) and the damage I take with the best armor I can find is too high.