So I go into the End, and halfway through my fight with the ender dragon (in survival) I realize that there are 2 ender dragons. In the screenshots you'll see that when I kill one of the dragons, there is still a bit of health left on the boss bar. Once I kill both, the boss bar is gone as normal, and the ender portal, both fountain and to the infinite end is there. In another screenshot you'll see that when one ender dragon is on top of the fountain, the other is desperately trying to get in too. So this must be a bug.