I had a world in 1.13.2 which I updated to the latest snapshot in 18w44a. Then, in 18w45a I entered the end and found that an ender dragon spawned naturally while I had already killed it. And same in 18w46a. These dragons dropped me the full amount of XP, so I think you should beware of that cuz it's pretty OP, it's basically free XP. But I think you have to get out of the game and enter again for it to work again, cuz in each log I only remember killing one dragon.