You can’t change your keybinds from default and have them work when mounted. Example: I switched my turn right and turn left to Q and E. When mounted, it still uses the A and D keys, which I now have bound to Block and Dodge.
THis is especially problematic with a bow. To turn my horse left, my bound key of E does nothing. So I use the old default of A, but while wielding the bow, it shoots an arrow, which is what happend when you try to block using a bow. It also turns the horse left.
I know it is confusing, but just try changing turn keybinds and you will see what I mean.
While on this subject, please make block with a bow a kick, like when not holding any weapons at all.