The bug
When using /setblock _x y z_ air destroy on a container (chest, dropper, etc) with items inside, the container is broken and dropped but the contents disappear completely.
I distinctly recall the contents dropping in 17w43b, and I'm pretty sure it worked in 1.12 as well.
How to reproduce
Put down a chest, shove some stuff in there, and target it with a /setblock destroy command. The chest drops, but the stuff inside is gone.
Workaround
As of 17w46a, it now works fine with /fill, but not with /setblock.
In 18w11a it says "Could not set the block" and breaks the block under the chest, dropping an item of the block below.