Ender Dragon can't die sometime with tag HealF:0

This command Instant kill Ender Dragon:
/summon EnderDragon ~ ~ ~ {HealF:0,DragonPhase:6}

This command don't kill Ender Dragon but it emit some effect when it's dying and it die after someone Inflicts 1 damage:
/summon EnderDragon ~ ~ ~ {HealF:0}

p.s sorry for my bad english