When the Ender Dragon flies to the portal, and is spawned in after another Ender Dragon, the Ender Dragon when flown back to the portal, it dies right away, at ANY health, even full health.
Video: https://youtu.be/4Jx-FrE1qMQ.
Seed: 3392903 on Superflat with 1 layer of Air, if that helps.
Environment
Java 8u25, Windows 10
Appears to be fixed in 15w33C.