Ender dragons spawned from commands have no HP bar. This doesn't make sense if it's intentional, since crystal-spawned dragons display bars correctly. Command-spawned withers also have HP bars, which shows that this is because only one non-command-spawned dragon can exist.
It's still annoying, though.
This actually affects all Minecraft releases, and I can't believe no one brought it up.
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We're tracking this issue in MC-82911, so this ticket is being resolved and linked as a duplicate.
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Duplicate of MC-82911, WAI. The boss bar is tied to the entire battle, not the mob; the wither always has a boss bar because the wither battle is only the mob.