glowing mob item outlining bug

I was playing around with the spectral arrow when I shot a spectral arrow at a skeleton and I noticed an obvious outline bug. I noticed instead of a outline of the bow itself there was an outline around the absolute borders of the texture creating a 'box' around the arrow. Afterwards I noticed this with all mobs that are holding items have the same affect (such as zombie pigmen or zombies spawned holding items). I also noticed this works on fancy graphics.

~~~_{HOW TO REPLICATE}~~~
1. Obtain spectral arrows.
2. Shoot 'em at a skeleton (or other item holding mob)