Bosses aren't affected by status effects (usually)

I was experimenting with the snapshot and wanted to know what an ender dragon would look like with the spectral effect, so I shot it with a spectral arrow, but it didn't work.

What I expected to happen was...
When I shot the dragon with a spectral arrow, it would glow with the spectral effect.

What actually happened was...
Nothing. It just took a bit of damage like it would with a normal arrow. Other arrows didn't work, and neither did splash potions. Commands worked only with the spectral effect. Nothing worked on the wither.

Steps to Reproduce:
1. Go to the End.
2. Shoot the dragon with a spectral arrow. Nothing happens.
3. Shoot it with an invisibility arrow. Nothing happens.
4. Throw an invisibility splash potion at it. Nothing happens.
6. Run

/effect @e[type=EnderDragon] minecraft:invisibility

The dragon still doesn't turn invisible.
7. Run

/effect @e[type=EnderDragon] minecraft:glowing

Interestingly, it does get the status effect.
8. With the Wither, not even the last command works.