The angle should probably be reduced to somewhere between 30 and 45 degrees in front, but I don't know if it's adjustable and Tydeus hates cav so it's not going to happen anyway. They'll probably just nerf cav again instead. Maybe horses should take damage from bumps too? Yeah, let's go with that.
I would assume that the inherent issue is that the angle of attack isn't something that the warband engine acknowledges. My guess is that as long as you hit the front hitbox(es) of the horse, then it will be reared, regardless of what direction the attack came from.*
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If the polearm user can hit the front of the horse from this angle before hitting its flank, then I guess this
could result in a 'legitimate' rear. Annoying for the cav player, of course, but I don't think there is any real way to avoid this.
*All of this is just complete conjecture of course, since I never looked at any code. I have learned that the warband engine is pretty stupid in most regards, so believing it can determine incoming attack directions would be a bad assumption. I could be wrong, and would kind of like to be wrong... only means that the engine is smarter than I think and that more can be done to improve issues like this.