Hitbox of riding Shulker doesn't extend

I spawned a Armorstand, ridden by a Shulker:

/summon ArmorStand ~ ~2 ~ {Tags:["elevator"],Marker:1b,Invulnerable:1,NoGravity:1,Passengers:[{id:"Shulker",Tags:["elevator"],NoAI:1,Silent:1}]}

Than I "opened" him but he didn't pushed you up. The hitbox stayed the same size. A Shulker, that isn't riding, extends the hitbox:

/entitydata @e[type=Shulker,tag=elevator] {Peek:100b}