Items in the hands of the Vex are incorrectly displayed

Summary:
Items in the hands of Vex are incorrectly displayed:
3D blocks are displayed very small and not placed in the hand.
2D items are displayed too large.

Steps to reproduce:

1. Summon a vex.
2. Give him 2D and 3D items.

   /replaceitem entity @e[type=vex] slot.weapon.mainhand 1 grass

   /replaceitem entity @e[type=vex] slot.weapon.mainhand 1 arrow