Custom mobs and normal Vexes drop equipped items with looting despite DropChances set to 0

I'm playing around with custom spawners for a combat arena in an adventure map. I noticed that my mobs drop items they have equipped, even if their respective drop chance is set to 0.

I've added the schematics file for a mob spawner, of which the mobs are affected by this issue.
The mob spawned is a PigZombie wearing a pumpkin on its head and a torch with fire aspect 2 in its hand.
They also have a healing potion in their chest slot with dropchance 0.6, which can't be seen. All the other dropchances are 0.
Two potion effects have been applied: weakness and invisibility.
I've seen them drop the healing potions, as intended, but the enchanted torch as well.
I've seen them drop their pumpkin when I started messing around with the spawner, but not (yet?) in the version I've attached.

Other info:

• I did not notice this in the 1.4.3 PRE.

/summon bat ~ ~1 ~ {NoAI:1b,ArmorItems:[{id:"stone",Count:1b},{id:"dirt",Count:1b},{id:"grass_block",Count:1b},{id:"glass",Count:1b}],HandItems:[{id:"white_wool",Count:1b},{}],ArmorDropChances:[0f,0f,0f,0f],HandDropChances:[0f,0f]}

/give @p minecraft:stick{Enchantments:[{id:sharpness,lvl:32767},{id:looting,lvl:32767}]}

Code analysis by Marcono1234 can be found in this comment.

As of 1.11, this affects Vex swords, since they have a HandDropChance of 0 on the mainhand.