Ender dragon auto respawn

After defeating the ender, if you logout and login your world, the ender dragon respawns with very little health. But the XP I got from killing the dragon the first time, and the items I used remained the same. I am using a OnePlus x running Android 6.0.1 with 3 GB RAM with 2.3 GHz snapdragon 801 processor and adreno 330 gpu.