Stray / Wither Skeleton / Pillager with Tipped Arrow in Offhand No Longer Shoot Tipped Arrows

The bug

In 1.10, Mojang made it such that, if a Skeleton holds a tipped arrow in its offhand, it will shoot that tipped arrow when attacking. This no longer works in 16w32a and 16w32b.

How to reproduce

1. Use the command:

   /summon minecraft:wither_skeleton ~ ~ ~  {HandItems:[{id:"minecraft:bow",Count:1b},{id:"minecraft:tipped_arrow",Count:1b,tag:{CustomPotionEffects:[{Id:18,Duration:100}]}}]}

2. Get shot by it.
   → Notice that you don't get Weakness.

Code Analysis / Fix

Code analysis and fix by Oval can be found in this comment.