Ender dragon's death animation resets when the chunk it is in is reloaded

To reproduce in singleplayer:

• Ensure that you are not in the world's spawn chunks
• /summon minecraft:ender_dragon ~ ~ ~ {Health:0}
• While the dragon's death animation is playing, /teleport @s ~1000 ~ ~
• /teleport @s ~-1000 ~ ~
• Observe the dragon's death animation starting over from the beginning

Note that while I'm only including the latest snapshot version in this report, this bug is absolutely ancient and there are a number of XP farms which take advantage of the fact that the dragon drops its XP in the middle of its death animation, rather than at the end. In survival mode it's also fairly trivial to get large amounts of XP by killing the dragon, quitting to the menu at the right moment, rejoining and repeating the process.

Also note that the dragon's position does not reset upon chunk reload like its death animation does, so exploiting this bug repeatedly will cause the dragon to rise far into the sky. (Panda4994's XP farm uses pistons to counteract this behavior.)