Wandering trader in invisible, but you are still able to trade with him, and his llamas are still on a lead. I found this randomly in my realm at night, and he holds items in his hand when you hold emeralds. He trades as normal and will give you items and xp when you give him emeralds. I presume he could have turned invisible with a potion but I didn't do that, and no body else is online. As you can also see in the picture, he doesn't have any particle effects on him, so I am reporting this as a bug.