The bug

Ender dragon hitboxes are inconsistent and have gaps.

• Right wing is 3 blocks high, the left one only 2.
• Gap between between head and body.
• Mouth hitbox is inside of head hitbox, likely because the head is shifted to the front causing the above described gap.
• Gap between tail and body.

How to reproduce

1. Summon an ender dragon

   /summon ender_dragon
   

2. Press F3 + B to show hitboxes

Code analysis (partwise)

Based on 1.12 decompiled using MCP 9.40 PRE 1

The wing sizes are inconsistent because the method net.minecraft.entity.boss.EntityDragon.onLivingUpdate() sets for one wing a height of 3, but for the other 2.