It may not be a bug, so much as the way the movement system is made conflicting with your movement restriction.
i dont know what to say REALY, (My only background knowlage is an intermediate grasp of LUA)
diagonal movement may have its own set of properties, and as for jetting, it may also be diffrent. (its all fluid and what not, unlike walking. makes me think vector3 inputs....)maybe you need to find a way to restrict any sideways movement input.