I have a mod by foxynotail called mini blocks, and I always go to wandering traders to see what mini blocks they have, and he didnt have a good block, and i killed them, while he was holding a invisibility potion, and it dropped the potion. It is kinda like the villager trades where if a villager is holding a trade, and you kill them, they drop it (I dont know if there is  bug report about that villager one). Have a great day.