Where to start? (this is all in the first initial Ender Dragon fight in survival)

If I quit the game and return, sometimes the Ender Dragon has disappeared, like completely despawned. Reloading the game sometimes brought it back. It seemed very random; sometimes it would be gone and reloading would bring it back and vice versa.

Sometimes the Ender Dragon was invisible, but could still deal damage, destroy blocks, and shoot at the player.

Finally, after defeating it, it didn't drop any XP.

(Also, I'm on 1.15.0.9, but the version drop-down didn't have that as an option.)