Mobs spawned from spawners or /summon never drop their equipment

Mob drops appear to be bugged when they come from a spawner. Have tested in 3 worlds at current time for a few hours. Have created a world in which this isn't happening. Looting doesn't change anything either.

What I expected to happen was...:
Bows and armor to drop.

What actually happened was...:
Only bones and arrows did instead.

Steps to Reproduce:
Go to skeleton spawner (presumably any kind however), kill as many mobs as you want, no rare drops will drop.