Currently, when riding armour stands, entities use the same behaviour of when they ride animals. By this, I mean the leg rotation is bound to a small movement to the left and right from the rotation of the entity.

Here's a GIF that demonstrates the leg movement issue, https://gyazo.com/6085f09b03d062de36433d6f1ed5c7e9.

Ideal behaviour would be for the player to rotate freely, without taking the direction of the armour stand into account.

This is due to Armour Stands inheriting the "EntityLivingBase" class, which is what is checked against before applying the effect. A simple fix is to exclude EntityArmorStand from this check.