The bug

If you kill a wandering trader while it is drinking an invisibility potion or a milk bucket, it has an 8.5% chance to drop the held item. (The chance increases with the looting enchantment.)

To reproduce

1. /give @p minecraft:emerald{Enchantments:[{lvl:255s,id:"minecraft:looting"},{lvl:255s,id:"minecraft:sharpness"}]}

2. /time set night

3. /summon minecraft:wandering_trader

4. Punch the wandering trader with the emerald while it is drinking the invisibility potion. The invisibility potion is dropped when the wandering trader dies.

5. /summon minecraft:wandering_trader

   Wait for the new wandering trader to become invisible.

6. /time set day

7. Punch the wandering trader with the emerald while it is drinking the milk bucket. The milk bucket is dropped when the wandering trader dies.