otherhand

What order did you stack them in?

Tested the same world as above, confirmed unchanged in 13w19a. The issue is still present.

Yeah, that's the trouble - it's not quite hostile enough, in that it sometimes makes such unnecessarily wide sweeps that it loses sight or interest in attacking the player most likely because that chunk isn't "active" anymore.
Often this results in the boss health bar disappearing as well.

For makers of adventure maps and other such creations, this isn't a stupid useless thing. Other mobs could be used to "corral" the dragon and keep it from just flying away or wreaking havoc. You could even put one on a minecart track.

No problem, hope this helps.

Added world containing offending spawner

I apologize, all fixed now.

This same issue is caused by moving around large quantities of lava, like in the Nether or some custom maps, etc. It seriously hampers the usefulness of lava.