Ender Dragon Despawns

I have gone through the End Portal to defeat the Ender Dragon, however shortly after the dimension loaded in, the dragon despawned. I respawned it with End Crystals, but later in the fight I died. When I came back to the End. The dragon disappeared again.

I don’t want to give up killing it because I didn’t get the achievement yet (Defeat the Ender Dragon).

The picture I sent, takes place after the first despawn.