Reached the end, destroyed all the towers, and was fighting the dragon. Got its health down to about half and then had to quit. I did not die, just quit the game to continue later.
When I continued again today the dragon's health was immediately back up fully. I've been fighting the dragon for the past couple of days, and now that I think of it, I think this has happened before as well.
Do damage to the dragon, after taking out the towers, log out and quit. Then later on continue the game and the dragon's health is fully restored!
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Relates to MC-105080
The problem seems to be that the legacy scanning which removes the existing dragon happens for updated worlds or newly created worlds as well.
Ok, now I have proof. The same game, just went in and hacked away at the dragon. Look how its health dropped in the first screenshot. Then quit, started again and its health was restored as can be seen in the second screenshot.
Now here the dragon's health is not really that much down, but earlier I had that health bar down much more, more or less half way, and when I went back to finish him off I had to deal with a regenerated one in stead ...