Description
There is a synchronization issue with the EnderDragonPart entity IDs between the server and client. Specifically, the ID of the entity parts on the client is one less than on the server. This discrepancy leads to desynchronization when hitting the dragon, causing the server to register a hit on a different part of the Ender Dragon than the client perceives.

 How to reproduce
1. Freeze the game /tick freeze
2. Summon an ender dragon /summon minecraft:ender_dragon
3. Step two ticks /tick step 2
4. Disable the ender dragon ai /data merge entity @n[type=minecraft:ender_dragon] {NoAI:1b}
5. Unfreeze the game /tick unfreeze

Observe how hitting the neck with a netherite sword increases the damage, while hitting every other part of the dragon does not.
Observe how hitting certain parts of the dragon are not registered on the server side.
Adding a debug statement in the hurt function of the EnderDragonPart reveals that the server and client disagree on which part was hit.

Expected behaviour
As the code indicates, only hitting the head should increase damage.
All hits should be registered on the server side.
The server and client should not disagree on which part was hit.

 
Technical Details
The issue lies within the recreateFromPacket function in the EnderDragon class. In this function, the first dragon part in the subEntities list gets assigned the same ID as the dragon itself, instead of being assigned the ID of the dragon plus one. Similarly, all subsequent part IDs are off by one on the client side as well.