Summon any entity with an ArmorStand entity riding it (for example a Zombie with an ArmorStand as passengers).
When you change the first entity's rotation, the second entity's rotation change, but not as much as the first.

This is only visual, because according to the NBT, the second entity's rotation did not change.

Steps to reproduce :

● First, summon a Zombie with an ArmorStand as Passengers :

 /summon Zombie ~ ~ ~ {NoAI:1,Passengers:[{id:ArmorStand}]} 

● Then TP the Zombie at the same place but with a different orientation :

 tp @e[type=Zombie,c=1] ~ ~ ~ 90 0 

● The Zombie has the correct rotation, but the ArmorStand does not. Its orientation changed a little bit but not fully.

Even if it would be really useful to have a NBT wich make the Passengers entity turn with the "rided" entity, this bug is annoying because it's glitchy.

Finally, we can see in the last screenshot that the Rotation NBT of the ArmorStand do not change. The rotation of the ArmorStand is only a visual glitch - wich come back even after deco/reco.

Here it is ! You can see the different steps in the screenshots.