The Ender Dragon can't take damage

The Ender Dragon can't take damage.I don't know why but I think this is because I am using a datapack to play Hardcore on mobile.I was trying to kill the ender gragon for the third time and the fire ball hit me and the I used a totem of undying and then the things gose crazy.First the dragon started being red and I was trying to hit him and the life started decreased and increased again and again.Then I left the game and joined it again ,the dragon was normally but when I was trying to hit him and I heard the sounds of damage but life did not decrease.Please can you repaire this bug!