Ender dragon consists of 9 entities in total
The bug
This happen since 19w08a.
When summoning an ender dragon with /summon command or spawned in end it will spawn 9 times all fixed to one.
How to reproduce
Spawn an ender dragon
/summon minecraft:ender_dragon
Kill the ender dragon
/kill @e[type=ender_dragon]
And the message appear "Killed 9 entities".