While I was playing Minecraft for Windows 10 when I hadn't my Switch, I did some tests about the Ender Dragon battle in creative mode.

1. When I go very far away from the main end island, the Ender Dragon is still there without be immobilized and invincible since 1.5.0.14. It's a good sign.

2.1 After killing the first Ender Dragon without problem, I summoned another Ender Dragon by using an impulsive command block so that it spawns in end cities islands by entering in the corresponding portal. After teleporting him, the Ender Dragon wasn't stuck, nor invincible, but he was very far from the main end island.

2.2a The Ender Dragon's teleportations in the end cities islands are very frustrating, because the Ender Dragon's boss bar no longer appears in the main end island. But then, the boss bar appears in the end cities as long as you're enough near from the Ender Dragon.

2.2b Because of the boss bar despawning in the main end island, players still think that the Ender Dragon is dead or stuck, but in fact, he flies quietly into the end cities islands.

In conclusion : I wish that you disable, for the Ender Dragon, his capacity to teleport in the end cities islands.