To reproduce:

• Summon a zombie like this:

  /summon zombie ~ ~ ~ {HandItems:[{},{id:"stone",Count:1}],HandDropChances:[1.0f,1.0f]}

• Try to get its stone:

  /loot give @p kill @e[type=zombie,limit=1]

• Result: Usually no stone.
• Kill the zombie manually.
• Result: Stone.