Not sure what's going on here, but i went to the end with hitboxes on, and noticed a bright green set of boxes flying above the Enderdragon, at first i thought there was more than one.

There might be.. i did /kill @e[type=minecraft:ender_dragon] and it returned 'killed 9 entities' something funky here >_>

[originally discovered this on my Paper 1.15.1 server, but went back to vanilla test world to verify bug source.]