Player's passenger rotation offset when a the horse

How I made the backpack

I've created a simple plugin that puts an armorstand on the player as a passenger, then I've set the armorstand's helmet to an item with the backpack texture (it has some offset so it displays on the player's back).

To make the armorstand face the same direction as the player I'm updating it's yaw to the player's yaw every time the player moves and every tick. Using this method: https://hastebin.com/mihiyucuvo.cpp

(I've also tried updating it to the horse's yaw when on the horse but it makes no difference, it returns the same value)

 

Problem

When the player is walking on foot everything works fine, but on the horse it looks awful with a really big rotation offset. The passengers get adjusted well if the horse jumps or the player punches.

 

Problem in general

Player's passengers have a rotation offset when on a horse.

 

Here is a video for easier understanding:

https://www.youtube.com/watch?v=LHNsj6poKlw&feature=youtu.be