Killing the Ender Dragon grants the "Free the End" advancement regardless of dimension the dragon was killed in

When you summon and kill the Ender Dragon in the overworld, the "Free The End" advancement is granted.

How to reproduce

1. Run the command:

/advancement revoke @s only minecraft:end/kill_dragon

2. Run the command:

/summon ender_dragon ~ ~ ~ {Health:1f}

3. Hit the ender dragon
You get the achievement even if you are not in the End.

Code Analysis can be found in the comment by ItsThatSpicymeme