I haven't tested in any other versions of Minecraft just 1.16.2 so I created a new world in 1.16.2 to like mess around and stuff when I went to the end I killed the ender dragon using /kill @e[type=minecraft:ender_dragon] it killed 9 entities It should just be 1 since there's only 1 ender dragon so that's kinda strange this actually might relate to  MC-105770 but I have no clue.

To reproduce:

Create a new single-player world with cheats and with creative mode then make an end portal when you get to the end just make sure like all the chunks all loaded for the ender dragon then kill it with /kill @e[type=minecraft:ender_dragon] then it should say 9 entities.