I am on the xbox version of minecraft better together. This happened on two seperate worlds all done legit vanilla. While fighting the dragon, when dragon breath is released the frame rate drops significantly. After dying with lots of dragon breath out I went back through the portal and the first time the dragon was missing with the home portal open and working and the other time the portal was working but the dragon was just floating by the ground flapping her wings. I have a video of that if you want it. No end city portal is open. On the first world I went to creative and respawned with crystals, fought her and it all worked fine - end city opened. This second world I want to keep legit so I'm currently poking around the nether looking for ghasts for tears for crystals to respawn. Pretty annoying.