When a boss is behind you or line of sight is obstructed, the HP bar does not display.
While this may not be directly a bug, it still breaks a lot for modded servers and mapmaking.
Reproducing:
[by rotating FoV]
1. Spawn a Boss mob (wither/dragon)
2. Turn around
3. The HP bar will disappear.
[by Line of Sight obstruction]
1. Spawn a Boss mob underground
2. Go to the surface
3. Look at the place you spawned the boss (you can use hacks, does not change anything)
4. Even though the boss should be (and is) rendered by your client, you do not see the HP bar.