So I was playing around with the Wither and decided to shoot a spectral arrow at him. I was in creative mode and got a Bow and a Spectral Arrow. I shot the Wither and noticed no glowing effect. I then shot another mob (Creeper) and the Glowing Effect showed up. I did it on the Wither and still no glowing effect when shot with a Spectral Arrow.

Replicate

• Have a Bow and some Spectral Arrows
• Shoot the Wither with the Spectral Arrow