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
Summon an ender dragon
/summon ender_dragon
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.
Related issues
relates to
Attachments
Comments

Confirmed for 1.13.2 and 19w13b
Confirmed for 19w14b
Can confirm in 20w51a.
Cannot reproduce in the latest snapshot via code, or gameplay:
[media][media]
If it would be helpful to find the exact fix version instead of resolving as CR, please let me know and I would be happy to look for it.